I have two of the K&B RV 15's. They went fine for FAI Combat (F2D). They were used successfully in RC Quarter Midget (15) pylon racing and could be bought complete with Perry carb. But in racing, being the fastest (with good pitting) is what counts, so they were squeezed out by the Rossi.
I took yet another look at the NCS (Denver rules) on 15 Profile Carrier;
Engine: 0.1525 cubic inch maximum displacement.
I think that one line says it all. Anything goes, within the swept volume limit. Muffler pressure, tuned pipe, crankcase, bar stock engine. Run what you brung. Subject, of course to the 70 MPH lid on the speed score. It isn't
exactly a "local" event. Unless amendments are posted, contestants expect the standard rules.
From a practical pint of view, current RC engines are generally sold set up for mufflers and muffler pressure and they won't work without it. At least not without some custom work. Look at it this way: The muffler pressure adds some boost, but the muffler also adds weight and reduces power, so it's a wash at best. Allowing muffler pressure
(as the rules passively attest) lets you simply buy and fly.