We have AM Supershifters with our more robust encoders (
which you've all seen) which generates a top speed (with the robot and everything on top) of 16ft/sec. I don't have exact specs, but thanks to our auto shifter (based on motor rpm) it accelerates pretty gosh darn quick.
Our robot has all of it's heavy stuff mounted on the left side, so we can take a four foot radius (left) turn at about 10ft/sec.
-q