I would love to see a 100% crop of the pine branch the red morph is perched upon. From here it is hard to tell, but the branch appears to be sharper than the bird. Possible missed/front-focus?
Regarding the use of teleconverters on APS-C. I use them. Hell, I've used teleconverters with both the EF 300mm f/2.8 L II and the EF 500mm f/4 L II on my 7D. I use both the 1.4x and 2x, and if Canon made a 1.7x, I'd use that too. Primes frequently have far more to offer from an IQ standpoint than sensors do. A lot of people complain about how "soft" the 7D is...that is true, sometimes...when using older lenses. Slap on pretty much ANY Mark II lens on a 7D, and that "soft" disappears, replaced by some of the sharpest detail you've ever seen. The Canon 18.1mp APS-C sensor is a good sensor...however it is a very, very high density sensor. If you use inferior glass with it, all the flaws OF THE GLASS are revealed. The only real drawback of the 7D is noise, and then, only at ISO settings above 2500 (and even then, with the increasing availability of advanced noise removal tools, such as Topaz DeNoise 5 (which has stellar random noise removal AND debanding!), high ISO noise is becoming less and less of a problem.)
To put some images behind my claims. Below are two photos of House Finches. One is the normal red morph, the other an orange morph. Same bird, otherwise, same size (maybe a slight size benefit to the orange morph) with the same amount of base detail...feathers, beak, eye. Both of these were shot at pretty much the same distance (around 7 feet...red morph maybe a few inches farther), ISO, and aperture, although the red one was up in a tree so my focal plane was shifted a bit, thus slightly blurring the top of its head and the back of its right wing. The body feathers and beaks are in focus on both birds. Both birds were positioned within the same rough area of the lens...slightly off center towards the upper left corner. Both full-scene images below are cropped to roughly the same area (few pixels difference in width and height).
Both photos shot with my 7D, ISO 400, f/6.3, in my backyard. The red morph was shot with my EF 100-400mm f/4.5-5.6 L IS lens with a full stop of additional light at twice the shutter speed (1/1600s, which should be an IQ advantage!) The orange morph was shot with a rented EF 500mm f/4 L IS II. Both lenses had AFMA adjustments for this body.
Here are the full images, scaled down to 900 pixels. Even at this level, you can see the difference in quality between the two photos can be seen. The orange morph is sharper and clearer (probably thanks to better microcontrast.)
At 100% crop (1:1 zoom, PIXEL PEEPING for all you pixel peepers!), the difference in IQ is beyond clear. The 100-400mm lens produces far softer results (even ignoring the slightly out of focus crest on the red morph). This kind of softness is what I've come to expect from the 100-400mm lens at less than f/8, and beyond f/8 diffraction again softens the image. (There is roughly the same amount of noise in both photos. It is more apparent in the red morph due to the increased lens softness, which blurs detail but does NOT blur noise. Clear, sharp detail tends to trump noise. The background in the red morph also provides a greater area of <= 18% gray tone, where noise becomes most apparent...the orange morph has a greater area of pixels > 18% tone.)
Scaled down to web size, the red morph photo is good enough. Most people won't notice the slight softness. From a print standpoint, I probably would not print the red morph photo, however the orange morph photo is definitely printable. It is not only printable, it could also easily be blown up two, maybe three times larger, and still be high quality, even higher quality than the red morph photo printed at original size!
I think the visual evidence speaks to itself regarding the sharpness and quality of, say, the EF 300mm f/2.8 L II lens (or any Mark II telephoto lens from Canon.) Canon is not releasing new lenses for the bulk of their lens lineup just for the heck of it. They are releasing new lenses to support their DSLR business for the next decade or two! The addition of IS or throwing in a Fluorite element here and there in the past were only minor updates on decades-old lens designs, and the impact to MTF charts was always minor. This is the first time since Canon introduced the EF mount that they are radically redesigning their L-series lenses to not only be lighter and more ergonomically ideal, but to significantly improve the MTF (resolving power/IQ) of each, as well as improve the AF circuitry to support much more advanced AF units that have found their way into the 1D X and 5D III (and, hopefully, the 7D II). In the past, even some of Canon's best lenses were still only in the range of 0.7 to 0.8 at best, and a bare few ever approached the vaunted 1.0 (the original EF 300mm f/2.8 L comes to mind as the prime example). The lenses released over the last few years, as well as those yet to be released or updated, all produce or will most likely produce MTFs well above 0.9 at best, and the Mark II telephoto lenses all approach 1.0 from center to nearly the edge.
I have no doubt in my mind that Canon is paving the way for 24mp+ APS-C sensors and 60-70mp FF sensors down the road. An extensive lens-lineup upgrade like they are doing is not just on a whim...they NEED the improvements to support the future DSLR, and a 24mp 7D II is probably only the beginning. Personally, I'm very much looking forward to a 24mp APS-C pro-grade camera from Canon. If they manage to achieve similar ISO gains as the 1D X has, it will be an astonishing camera indeed. At 10fps w/ a 61pt AF system on the 7D II, Nikon...who as of yet has not shown much interest in updating the bulk of their lens lineup to support their
24mp APS-C sensors or 36mp FF sensors, won't have anything that will solidly compete with it!