I'm going to offer advice on a different level.
My advice is to use this as an opportunity to be a better photographer.
It is very simple if you ask me, and I've said it before but you must learn the "exposure triangle". The relationship between aperture, shutter, and ISO and how one impacts the other.
Shooting is really based on light, you must either use available light, create light, or combine the two.
As you have seen here flash is not what you want to do, most performances will not allow it. This is not the reason to not use it, the reason is it ruins the impact of the performance, think about it...you are trying to capture what the audience saw. They did not see it all lit up with flash. They saw creative lighting, shadows, and vivid color. You need to capture that.
Now you have to ask yourself another question...hmmmmmm...no flash and most performances are dark. So how do I shoot low light with no flash...well now we are back to understanding how to properly expose an image.
How do I make the camera more sensitive to light?
I can use a slow shutter speed - How slow? I don't want blurry images!
I can open the aperture way up - Now my depth of field might be off or my lens only goes to f/4.0
I can shoot with a high ISO - Where does my specific sensor ad so much noise that I can't save it in post processing?
You need to understand the limits each one of these creates and then and only then can you figure out a balance that gives you the best quality image available.
What is the perfect lens...well it is impossible to answer that question without knowing the exact size of the venue. Will you be able to walk around or must you stay in one spot? Is this spot close to the stage or in the back? What happens if I'm told one thing and then I get there and it is another?
I think you get what I'm trying to say...
I no longer ask....what lens do I need for a specific type of shooting? I might ask other questions like...I'm shooting a car race and I'm curious what shutter speed do the pros who shoot this tend to use? I'd ask this with the thought of wanting to have sharp images but be able to show motion. I would know I need a variety of glass because where I shoot from and the type of shot I want will vary.
I hope this makes sense......learn how to be a good photographer and you will be able to answer your own questions.....very quickly!
I agree on shooting the dress rehearsal, you can see in my thread of the Wade Kilgore CD that I actually had a thread in the people section where I went out the day prior to our shoot in the same conditions and did a dry run. This way I knew what lens I'd use, what to expect with the lighting and the best spot to shoot from. Since I did this I'm now happy with the results.