In extreme low light, I switch to primes, because I find that f/2.8 does not quite make it for many shots.
I use my 35mmL, 50mm f/1.4, 85mm f/1.8, and 135mm f/2.
I could use a f/2.8 lens at ISO 12800, but I prefer a lower ISO setting unlless I'm really in a bind. As it is, a few shots will always end up at f/1.4 and ISO 12800 thats ok when its just a few.
I tried using my 24-105mmL for a few images last year, it was a exercise in fuzzy images shot at too low of a shutter speed. My subjects move, so IS doesn't stop motion, and flash is banned in a theatre.
I'd recommend sticking to a minimum of f/2.8, but you know best how much light you have.
Here is one when I was trying out a D800 and the f/2.8 lens was as fast as I had. Its ISO 6400 with a ton of nr, and losing a lot of detail. Fortunately, the D800 had a lot of detail, so it wasn't too bad.
My 1D MK IV at ISO 12800 and f/1.8