Re: what lens?

I would be wary of any lens that does not have constant aperture. Remember on that Tamron 55-200mm f/4-5.6, you will be at f 5.6 on the long end (zoomed all the way in), which is where I would assume most of your shots would happen, unless you can't get really close to the stage.

1) look into renting lenses for special situations like this. A Canon 70-200L can be good for these situations.

2)Unless you have a gigantic (in size and price) lens and if you plan on getting anything close to a close up or even a medium shot, you are going to need to be much closer to the stage. Otherwise you are looking at a gigantic lens (in size and price) Picture sports photographers on the sidelines of a professional football game, and that is what you would need.

John