the contrast detection autofocus in liveview works with f8 lenses - and it seems faster and more reliable than normal phase detection autofocus with a 1.4x and pins taped on the 100-400 and 400 f5.6 - have not tested accuracy and repeatability but in theory it should be fine
i've been testing a 7D with 400 f5.6 lens searchin for a combination that will allow larger than 13x19 wildlife prints at 600mm + and is compact and lightweight. i've rented a 400DO on a couple ocassions and concluded that unless you really need f4 (which you don't very often any more) it is not worth carrying and certainly not worth the price. the 400 5.6 has equal center resolution and is far sharper at the edges than the DO - at f8, there is no difference in center and edge resolution
the other reason for trying the DO was it's ability to autofocus at (almost) reasonable speed with a 1.4x - unfortunately this isn't useful as the IQ of the combination is garbage. the 400 5.6 on the other hand is at least as compatable with the 1.4x optically as the 300 2.8 and 500 4 - good enough for 13x19 prints at 896mm. i did have to crank in a quite large micro-adjustment for the 1.4x
i've been testing and agonizing over the combination all weekend because it does not have the big jump in resolution over the 40D suggested by the pixels - or even the more modest improvement i expected - i measure about 10% improvement. i did a baseline test with 100 IS and the 7D had about 20% more resolution than the 40D (and about 25% less than 5D2) - it appears to me that there are very few lenses that are good enough to realize the benefit of the 7D's high pixel density
the improved ergonomics of the 7D is impressive, but it must be that Canon's chairman doesn't like mirror lockup - an amazing omission given the number of buttons and degree of customization possible - of course i'd like it to autofocus at f8 like my old EOS 3, but since it will be on a tripod at 896mm, the contrast detection will do just fine
now that i've decided to keep it, Canon will finally announce a 400IS f5.6.
