Richard,
Shooting indoor, at moving target with no flash. It does not get any tough than that !!!
A teleconverter make matter even harder, it forces you to have smaller aperture and faster shutter speed
My G15 will is suppost to be here today I hope, but my advice may not be worth much.
My suggestion is go to one of the dance rehearsal, get close so you can use your 1.8 aperture, and take some picture and when the show comes, give up on taking photos ang just enjoy it instead
Please post some picture on how your photo's turn out.