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After I read your post, I looked at the Canon Website for their lens lineup. I didn't find any telephoto zoom better than 2.8 and the fastest telephoto prime is the 2.0 135mm. Did I read your post correctly that you have a Canon 200mm 1.8? Or is it a different brand or obsolete?
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I consider the Canon EF 200mm f/1.8L prime lens one of the 3 best lenses Canon has ever produced. Unfortunately, for some reason, Canon discontinued producing the lens last year. Now, you'll have to look to the used market to obtain one - pricing seems to run from $2500 - $3500.
I use the 200 f/1.8L for most very low light situations. Such as this one, shot today at a municipal hockey facility, no flash, at 1000 ISO, f/1.8, and 1/640th, with a 1D:
http://www.pbase.com/image/26714187 .
While I have not ever shot gymnastics, I wouldn't think the lighting would be a lot different. The Canon 1D can go up to 1600 ISO with reasonably good results. That's two-thirds stop improvement beyond the above shot. And then, perhaps you can drop down to 1/320th shutter speed, which is a full stop improvement beyond the above shot. Total improvement with different settings, same lens, same body, is 1 + 2/3rd stops, which is almost a cut in light by 1/4th.
If you live in a major metropolitan city, you should be able to rent the 200 f/1.8 for a reasonable fee. Here in the Dallas area it costs about $35 for a week day / week end to rent. Well worth checking out. Just an amazing lens.
Best regards, Bill