For what it's worth, I don't think the C rating will have a noticable effect on racing. We've been running minis on Tuesday nights. I run two packs, one rated at 3400 mah and 20C and the other at 3000 mah and 25C. Logic says the 25C pack should be faster, though on the track I can't feel the difference and I can't see the difference on the heat sheet. We'll need to get these power systems on the track before we worry too much. The pack can be spec'ed if need be later.
Neccros, what are you running now? Pretty sure just about any of the gear you'd run in RCGT, 17.5, or 13.5 would be sufficient for 21.5.