I gave up on the f/2.8 zooms for now, I've had most all of them, and in low light they do not allow fast enough shutter speeds for moving subjects, even with my 5D MK II. They are fine in well lit sporting fields, its theatres or dimly lit buildings that are just too dark.
So I use my 70-200mm f/4 IS outdoors and use fast primes indoors. Since you have 50mm, a 28mm f/1.8 , and a 85mm or 100mm prime might be the next step. You can buy the 85mm f/1.8 and a 28mm f/1.8 for about the price of the 70-200mm f/4L
As soon as you get dancers in almost darkness, it takes a fast prime, high ISO, and slower shutter speed than I'd like to capture a image. Lots of motion blur here.
5D MK II ISO 3200, 135mm L @f/2 and 1/125 sec