RE: Bob and his P51 (and my DAGO)
RPM gain depends on the setup.
Usually it is along the order of 500-600 rpm, but the biggest difference is torque. You can usually go up one prop size from the stock muffler, and keep the same rpm.
On your setup with the 55 you will probably want to run a 10x8 or 11x7 prop.
When you have a chance, let me know what prop you have, and the rpm you are seeing.
Also, be sure to launch the engine set down from peak rpm about 500-700 rpm. It will unload in flight a bit, and will need the fuel.
Bob