200mm & 800mm do not have Power Focus. I have both and the feature is not present. I use 300/400/500/600 Series II Super Teles and they have this feature. In fact the 400's in my drybox with the 200/300/800.
You and I may be thinking of other features. When you say "Power Focus", I am thinking of the focus preset feature. Set a certain focus, save it, then have the ability to return to that focus with a twitch of the focus preset ring. The lens will return to your saved focal plane automatically. I believe this setting can be set with fast or slow focus return, allowing for nice, smooth focal transitions when recording video.
The 800mm f/5.6 L IS DOES have that feature. I know for a fact the Mark II 300mm, 500mm, and 600mm lenses have that, too, as I've used all of them as well. The only one I have not used is the new 400mm II, however based on photos of these lenses, it appears to have all the same controls as well. According to the US manual for the EF 200mm f/2 L IS lens, it also has the focus preset feature. See page 8 in the following manual:
http://gdlp01.c-wss.com/gds/0/0300003440/01/ef200f2lisusm-en.pdf If this "focus preset" feature is not the same feature as "Power Focus", then I am not sure what you are talking about.
Printed user manual states 5-stops of IS.
I think there are differences between UK and US markets.
On the US web site for Canon, it clearly states 4-stop:
This ultra-fast telephoto, a new member of Canon legendary L-series lenses, has totally new optics to provide better image quality. It uses fluorite and UD lens elements for excellent chromatic aberration correction and consists of 17 elements in 12 groups. The built-in Optical Image Stabilizer gives it up to 4 stops of stabilization correction. The inner USM and optimized AF algorithms result in fast and quiet autofocusing, and the circular aperture can even produce beautiful out-of-focus images. This ultra-high-performance lens also improves its durability - better dust- and water-proofing. The EF 200mm f/2L IS USM is outstanding for many available-light applications, including indoor sports, theater work, fashion, and candids at events.
The exact same lens
on the UK site is listed as 5-stop:
Five-Stop Image Stabilizer
Canon’s five-stop compensation Image Stabilizer technology allows the use of shutter speeds up to five times slower with no perceptible increase in image blur. Automatic panning detection automatically turns off the Image Stabilizer in either the horizontal or vertical direction when following moving subjects. Tripod detection automatically switches off IS when the camera is supported.
Either way, I don't believe Canon has DIFFERENT IS systems for both markets. It is the same IS system, regardless of whether it is marketed as 5-stop in the UK or 4-stop in the US. It's all just marketing. It is also very likely that the chances of getting up to five stops of hand-holdability at 200mm on a system designed for four stops at 800mm is just a happy coincidence. At 200mm your angle of view is much wider, so camera shake is magnified by a much lesser degree. As I've mentioned, most reviewers seem to get around 5 stops or more of hand-holdability anyway with ALL of Canon's lenses that use the 4-stop IS system. It's all just marketing...but that isn't my point.
My point is, Canon's current IS system was
originally designed for the EF 800mm f/5.6 L IS lens. The technology was innovated
with that lens. It is now simply being propagated to the rest of Canon's line of modern white telephoto and supertelephoto prime lenses as part of their current refresh. There haven't been any significant advancements in image stabilization systems in the 3-4 years since it was first introduced. It was over a decade since Canon first introduced their 2-stop IS system before they first introduced the 4-stop IS system.
You are taking about exposure. I am more interesting in freezing a football player in full run vs having a slightly blurred football player in full run. You obviously do not shoot action photography where a fast aperture is ideal and often required for shutter speeds of 1/1000 or faster.
Your now claiming you are talking about exposure in terms of quantity of light on the sensor. I know what your talking about, and I am saying it does not matter. Not when we are talking about a third or a half or even two thirds of a stop. It would matter if we were talking about differences in exposure of over a stop. If we were debating the merits of exposure value (as determined solely by shutter and aperture, i.e. quantity of light at the sensor), the difference between an f/2.8 and an f/5.6 lens is very significant. But we aren't talking about that.
What you are actually talking about, and the point I was always discussing, is the ability to achieve a faster shutter speed. The argument was that only an f/4.5 lens would allow you to achieve an additional two thirds stops faster shutter speed. I'm saying that is wrong. ISO is one of TWO factors that affect shutter speed (which is what we are really talking about here, not exposure value.) I shoot birds, which most is most definitely action, and most definitely requires high shutter speeds. I'd argue that I need higher shutter speeds to freeze the constant micro-motion of a bird than is necessary to freeze a running baseball player or leaping basketball player. Even at 1/1600th, I often capture motion blur of small passerines...even just turning their head, they can move half a centimeter or more in a tiny fraction of a second.
When I need a high shutter speed, I either open up the aperture, OR I increase ISO. BOTH allow the use of a faster shutter. In the context of the current discussion, your statement is that you would only buy a new 800mm lens if its max aperture was bumped up by a third or half a stop,
so you could get faster shutter speeds. If THAT is your argument for a new lens, then you don't understand the purpose of high ISO, or why Canon put more effort into achieving cleaner high ISO settings in the 5D III and 1D X instead of improving DR at the lowest ISO settings. Increasing the ISO setting by a third or half a stop is TRIVIAL...you won't notice any additional noise, especially with one of Canon's newer DSLRs. If you are at 1/1000s at ISO 1600, and you need 1/1250s or 1/1600s, you don't
absolutely require a lens with an f/4.5 aperture. You can jack up ISO to 2000 or 2500, and achieve the necessary shutter speed. Your EV will be different...lesser by a one or two thirds of a stop...but again...trivial difference, it doesn't matter.
If you were arguing for an 800mm f/2.8 lens...that is a significant difference. Bumping ISO up by two stops will probably have a visible impact on noise, and your EV would change by a significant amount. But we aren't talking about that. We are talking about changing the max aperture of an 800mm lens from f/5.6 to...f/5, f/4.8, maybe f/4.5? Not worth it. Not worth the cost, when you can get that extra few thirds stop
shutter speed with a trivial increase in ISO (which, for all intents and purposes, is free...or maybe requiring the sale of a current camera and the purchase of a 5D III or 1D X.)
High-end white telephoto prime lenses with fractional f-number stops.
http://www.canon.com/camera-museum/camera/lens/ef/data/telephoto/ef_200_18l_usm.html
http://www.canon.com/camera-museum/camera/lens/ef/data/super_telephoto/ef_500_45l_usm.html
You keep bringing up decades-old lens designs. I mean, 20 years or older. I don't see how those are relevant to a discussion of MODERN prime telephoto lenses...or even the last generation...? I mean, you might as well drudge up some old FD lens designs while your at it...I'm sure we can figure out a way to blend them into the conversation somehow...and you could probably find some f/6.3 max aperture ones...
If you can't afford a $80,000 lens then you're not the market for it. 
Erm, its not an $80,000 lens...its a $13,500 lens. And, as it stands, I AM in the market for it.
Actually, I'm in the market for the EF 600mm f/4 L II and a 2x TC, which is just as good if not better than the 800/5.6. Which is also a $13,000 lens. You get what you pay for. I WANT the EF 600mm f/4 L IS II. It is pretty much the pinnacle of lens design right now, in my opinion. Superb IQ wide open, very light weight, optimally balanced for excellent hand-held usability, incredible image stabilization (which seems capable of at least five stops in my personal experience with this lens and the and 500mm f/4 L II, and over five stops in my personal experience with the EF 300mm f/2.8 L IS II).
I'm honestly not sure of the practicality of an 800mm f/4 L IS...the thing would be a beast, would be very heavy, would be difficult to balance with such a huge front element, etc. etc. I'd be curious to see how an 800mm f/4 DO lens performed...I believe such a lens might be hand-holdable and decently balanced. I'm not sure a DO lens can perform as well as a standard L series lens...but, maybe 15 to 20 years from now, we might all get a chance to try one out.
