Chuck,

1. Could you give me the formula for mach speed versus RPM for given prop lengths?
Warp has 2 pretty good calculators here: http://www.warpdriveprops.com/propspd2.html

They are generic, not specific to Warp propellers and a "rule of thumb" only. I do not know of anyone that publishes specific calculators for any specific prop.

2.Are the performance percentages a percentage of the mach one speed?
No, but there is certainly a correlation, 65% of tip speed does not equal 65% efficient. That is specific to each prop design, hp/torque/rpm and pitch and can only be determined by testing the configuration.

You can get some pretty good ideas though by changing the diameter, pitch and/or rpm in the calculators shown to see the theoretical change in performance.

As others have mentioned, the 1st thing I would verify is rpm readout accuracy. I would also check to see that you are getting full range on your throttle cable at the carb and go from there.

Greg