the 100 doesn't work with the 1.4x (unless you use a short extension tube), but that's not really a big problem.
Good point; I forgot about that.
aside from lack of IS, the downside of the 200 is lack of reasonably close focus, but it's about as sharp and distortion free as anything Canon makes - IQ pretty similar to the 100 IS (even though it's measurably better than the 70-200, i don't expect anyone to notice the difference in a large well-edited print)
Yes. And you got me thinking, so I did a quick sharpness test, 70-200 f/4IS vs. 200f/2.8, both at f/4. Not fair? Of course not, but we all know which lens is sharper, the question I had was how much? So here are the two 100% crops.
3-series Gitzo
BH-55 ballhead
MLU w/ delay
Focus w/ Live view at 10x (I had to do this twice; the first time the 200 wasn't focused good enough... had to get my reading glasses)
Sharpening at 0 in LR for both
White balance on the x-rite card
Lightened the exposure +0.35 for 70-200 to get similar brightness
The zoom is pretty damn close, and that's wide open...