I have had the 100-400 for about 16 months and don't use it often but thing it's an excellent lens.
However I recently had the opportunity to photograph a concert (the Yeah Yeah Yeahs at the Hollywood Bowl) and rented the latstest version of 70-200. I brought 2 other bodies and lenses and my 1.4 and 2.0 TC's.
As I happened there was also a film crew there so the places one would normally get to shoot from, the pit, the side of the stage, etc. were off limits. All of the photog's were relegated top the sound board area which at the Hollywood Bowl puts you just about in the place where 400mm get's you reasonably close shots.
On top of that instead of the bands normal light show, including spotlights on the band members, the stage was lit for the film crew. It made for fine viewing but was very poor lighting for fast-ish shutter speed still photography.
I'd heard the 2.0TC worked well with the 70-200 so I threw it on and shot away, with 90% of the shots I took being with that combo.
I was kind of blown away at how well, especially in the challenging lighting, the combo worked. I'm not in a position to purchase one right now but would have no hesitation using the 70-200 with the 2.0 TC for wildlife.
Some samples (all cropped). The last one was picked up by Vogue for an article on the singer's (Karen O) costumes (note she'd walked onto a walkway in front of the stage allowing for tighter shot).
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