The low light is the killer in your scenario there, Matthew.
When I was in Canada last year we went to the ice hockey, and it was no problem to get good shots from halfway up the grandstand even to the other end of the rink (though my camera has IS
).
However, for what it's worth here might be the place for some advice my brother (the real photographer in the family
) gave me before going to South Africa last for a Vertical Rescue Competition. The Competition was being held in the Sun Dome at Jo'burg. It's a huge dome 200m diameter and >60m high. Lighting conditions quite dim for photography - flash was a must. His advice was set the camera on manual with 1 stop more aperture than required for the flash... I was using an aftermarket Ham'n'Eggs flash on my old OM-1 and 400ASA film. It Worked a treat!
Under normal conditions the flash had a max range of 5m I think, but my shots were good out to 15-20m. Ofcourse the action in a vertical rescue comp is nowhere near as quick as gymnastics, so this may not be of use to you.
Considering I didn't ask him for the advice, he obviously made some excellent assumptions about the conditions I would find when I got there and offered it straight up.
The only other option I can think of is buy a big flash! ... or a camera with really high ISO
...
Al.