Gave my new (slightly used found on e-bay deal) 400mm f5.6L a spin and tested against my not so handy-dandy 70-200 f2.8L IS +2xTC @400mm. Shot on a 1D Mk II, Large jpg mode, ISO 200, lenses mounted on a Manfrotto tripod.
First the large picture with the close up circled in red, then the closeups shot at f5.6. f8 and f16. Interesting that f8 seems to be sharper on both lenses than wide open and f16.
Wish I had the 100-400 f3.5-5.6L IS for comparison. Aw, shucks, I wish I had the 400 f2.8L IS and the 400 f4 DO IS for comparison, but I need a winning lottery ticket for that.