With that much fuel through it you should see max rpm out of it! They may have toned down the compression with the new piston but why....
I would do a basic check of the whole motor. Start with the valve lash. You should check this every few hours of run time. Sounds like you have enough fuel through the motor to have checked this a couple of times. This is one of the main reasons for performance degradation.
Check all bolts for tightness. Pressure loss through the crankcase is also a cause.
Double check the regulator for correct installation and holes. Even the manufacturers can screw up.

Hope she feels better soon!