I own two lenses that I would like to use with the teleconverters. My Canon EF 70-300 IS USM f4 lens and my Canon EF 70-200 IS USM f4L. Now I know the teleconverters work best with F2.8 or faster glass but I don't own a f2.8 zoom lens.
My choice would be the 2x as it would make my 70-200L a 400mm and my 300 a 600mm lens, but I am not sure what the lower light would do for the autofocus and such. The 1.4 would be OK but then I would have to use my 300 as a 400mm. (420mm to be exact)
My question is, how are the optics with these teleconverters and what exact model would you recommend?
OK, so firstly, i'm presuming you have the 70-300 IS USM f/4-5.6 non-L (although these answers will be exactly the same if you do have the L).
Now, there are two types of teleconverters, reporting and non-reporting. When you mount a 200mm f/4 lens with a 2x Teleconverter, you get a 400mm f/8 (because that's how t/cs work). Now, a reporting-teleconverter will tell the camera that you have a 400mm f/8 lens, a non-reporting t/c will tell the camera that you have a 200mm f/4 lens.
Mounting a 2x Teleconverter on the 70-300 will bring it to f/8-f/11, and mounting a 2x T/C on the 70-200 f/4 will give you a 140-400mm constant f/8.
Now, if you use a reporting teleconverter, using a 2x t/c, unless you have a 1D, 1Ds, 1V, or 3, your autofocus is NOT going to work, at all. If you do have one of these bodies, you will get autofocus only at the centre AF point (better than nothing). Using a reporting t/c on a 1/3-series, the camera will slow the Autofocus motor down so that it's more accurate.
If you use a non-reporting teleconverter, the camera will still think you have an f/4 or f/4-5.6 lens. So it will happily *try* to autofocus. Whether it succeeds or not is debateable. I use my 70-300L with a non-reporting Kenko 1.4x Pro300DG sometimes. On my 7D, the autofocus tries and fails to AF a lot (normally I leave it to MF). On my EOS 3, autofocus *should* work, because the centre point is f/8 sensitive, and the lens + t/c is f/8 at the long end. But because the camera doesn't know about the teleconverter, it runs the AF motor at full speed and most of the time it misses focus and has to wobble about a bit before it locks. If you're using a 2x t/c, that problem will be even worse, and you're probably better off just using MF anyway.
So, what should you get?
If you have a 1D or 1Ds (not 1DX), you can try to use a 2x reporting T/C, and you may get AF at the 70mm end of the 70-300, or on the whole of the 70-200 f/4.
If you have a lesser body, I'd be sticking with a 1.4x t/c on the 70-200 only if you want AF, anything else you'll have to MF.
As for IQ, a lot of people just steer clear of 2x teleconverters as a matter of course. Some are definitely better than others though.
I've got the Kenko Pro300 1.4x DG teleconverter, it's a non-reporting t/c for 7-pin lenses like the 70-300L (and 70-300 non-L), but it reports for 10-pin lenses like 70-200 L and 100-400L. There's only one above it in the Kenko range, the newer DGX version (which afaik is a reporting t/c all the time, someone feel free to correct me on that).
Image quality is always subjective, I think it's fairly good with my 70-300L attached (I used to own the 70-300 non-L, and I didn't like the IQ at 300mm even without a teleconverter. If you think that it's ok, you may be ok with using a t/c on it too).
As for Tamron, someone else can answer on that, I've never used them...