Quote Originally Posted by abrama94
My big reason to upgrade is that with the 2X extender (for high school sports andwildlife)the image seems to be better than the Mark I and compares favorably to the 100-400mm (less so but still comparable to the 400mm f/5.6).

I do agree that the MkII seems to give much better results with the extenders than the MkI. But, be a little cautious about judging the performance with extenders across the zoom range - at least in Bryan's tests, the extenders are only tested at the longest end of the zoom. In other words, you can't see what a 70-200mm + 2x looks like at 200mm (i.e. with the zoom ring set to 100mm) - performance may not be consistent across the zoom range. Unless you're planning on swapping the t/c off and on during a hike or a game (not easy), I'm honestly not sure how the MkII as a 140-400mm f/5.6 zoom would compare to the 100-400mm f/4.5-5.6 when each is used across their range. I'd actually really like to know the results of such a comparison!