I have to put my two cents in:
If you do shoot for a fast robot, make sure that everything is mounted very securely. Just the other day I was driving our practice bot (last year's chassis) around on high gear, which tops out around 12 ft/sec. I went full speed foreward, then overcorrected (hey, I'm a programmer, not a driver) and threw the battery out the front of the 'bot. As I said, make sure it's secure.
And, make sure your autonomous code isn't programmed to use high gear...
JBot