If you're looking for lots of speed over decent torque, you could use smaller wheels and gear the motor up with belts, gears or chains. Or overvolt the motors a bit.
Drill motors are very speedy on 12cm wheels, or if you put a layer of bike tyre over the 10cm Robo Challenge wheels, you get something near to 12cm diameter and you gain some additional grip from the tyre.

EDIT: I wrote this thinking of a horizontal thwackbot, but having seen Calum's post, if you want a Saint-style overhead thwackbot, using the suggestion of smaller wheels may be less practical. If you're going with the overhead design and want wheels of 15cm or above, you may have to look at gearing down the drill motors or using different motors with a more substantial built-in gear ratio (windscreen wiper motors etc)