MJD, when I'm shimming the head I'm not looking for a peak RPM difference, I'm looking for an engine behavior difference. You simply can not have an engine that chews up and spits out the elements. The combustion chamber and piston should have a sooting glaze finish, not sandblasted or pitted. The rod ends, crank pin and bearing get hammered from too much compression. A tuned pipe effectively raises the compression br cramming more air into the chamber. If you are running a racing engine with maxxed out breathing ability and unloaded rpm potential, you want to work the compression from the lowest that will barely run up to a point that seems worth trying. From there, you can almost throw the tach away and only pay attention to the health of the glow plug and the times you are getting from the plane.
For the topic of this thread which is about the sequence of adding nitro and shims, [not basic engine running hygiene], it's the same deal...start out with enough shims to make the glow plug survive a bench run or 2, then go fly it and take your tools with you to the field for further fine tuning.