Combines transport with rwd turned off, and they go the same speed as a similar 2wd model.  With rwd on speed is reduced about 30% because of the oil diverted from the main motor to feed the rwd motors. 

The combines I have been in all had tapered roller bearings just like everything else on the place.  I don't see why they would be speed restricted at any reasonable RPM...similar type bearings turn at engine speed in a transmission.