What max RPM do the props turn? Same for single and twin motors?

From : Don Stackhouse

Yes, both about the same, other than some minor differences due to the greater voltage loss when feeding a second motor (although generally we're using larger cells on the twins, so there is essentially no voltage difference). Both singles and twins typically run about 5000 to 5100 rpm at full throttle.

Don Stackhouse
DJ Aerotech