Indoor or outdoor stage performances and what's the lighting like generally and how much motion/movement is there? Only you of course can make an assessment on that.
I recently shot my kids during a indoor stage performance at their school, which had variable lighting depending on what was happening on stage. I used my 60D and my 70-200 IS II 2.8 and I was able to get some good exposures wide open at 2.8, shutter speed ranging 1/125-1/200, and 3200 ISO (not great on the 60D, but mostly salvageable with Nik's Dfine for my family purposes). This manged to stop 95% of the motion, there was nothing too fast and furious going on. When the lights really dropped down the shots at 1/25-1/60, almost all ended up in the trash due to motion blur from the performers. I think the FF is a way to go for the performances, as I found the 70-200 was a little tight on a couple of occasions due to my crop.