300 f2.8 IS L II plus 2x Mk.III better than 100-400 f4.5/5.6 IS L?

On my 50D, here's my assessment:

ID: 300 II + 2x TC III better than 100-400 @ 400

AF: 300 II + 2x TC III similar to, or slightly slower than, 100-400 @ 400; it also tends to hunt a little more than the 100-400.

The AF results may be different if you use a different body though (this is why I am eagerly waiting for the 7D II, but that's another story).

In short, the 300 II + 2x III is a completely usable combo.

Here's a picture I took with the 300II + 2x TC III @ f/8, 1/500s, ISO 320:

Red-shouldered Hawk

Here's another one at f/7.1, 1/500s, ISO 200:

Red-tailed Hawk

--
Howard
cameras: 5DII, 50D, D60, Rebel 2000 (film)
lenses: 17-40 f/4 USM L, 24-105 f/4 USM L, 100-400 f/4.5-5.6 IS USM L, 24 f/3.5 TSE L, 35 f/2, 50 f/1.4 USM, 100 f/2.8 IS USM Macro L, 300 f/2.8 IS USM II, 430 EX II, Kenko Pro 300 1.4x TC, EF 2x TC III
personal website: http://www.travelerathome.com
blog: http://travelerathome.wordpress.com