Jeff,
I've been using the Sigma 70-200 F2.8 with a Sigma 1.4 TC and have found that you just can't hand-hold this combination at maximum magnification and get decent results. As David says, it magnifies every movement even at high shutter speeds.

I've propped the combination on fences, buildings, against cages (at the zoo) and gotten fair results. But for sharpness you just can't beat putting the rig on a sturdy tripod with a good ball head. I even use a cable release to minimize vibration. And with that combination, I get acceptable results up to 11X14 prints.

Jim