A little engine history and faq.
Any engine run without a muffler WILL have more rpm and power than run with a standard muffler. This has been a proven fact for several decades.
Mufflers came into wide use due to areas for model activity were being lost due to noise. Mufflers made things quieter. Non-modeling folk were happier. RC folk lost less places to do their activities.
Due to loss of power with standard mufflers, tuned pipes became more popular for everyday craft of all sorts and no longer termed for competition uses only. Not only did pipes increase rpm's, but it did it quietly.

Pressurized fuel systems was the only thing to use to be consistant in CL and FF aircraft competition since the 1950's. This was always crankcase pressure ( much higher pressure than exhaust pressure) and in a sense, was a poor mans supercharger, as the fuel was forced into the engine and easily consumed due to the use of VERY large VENTURIES (intakes) and humongous porting of the engines.

RC folk tried this route to increase performance but quickly determined that the pressure was too great for CARBS to handle (excluding PDP modified engines, but that's a story in itself). It was also determined that exhaust pressure was more than suitable, as it is continuously variable( the higher the rpm the engine is run at, the higher the pressure created). Though it did not increase top rpm as much as they had hoped, it did help greatly in the throttle in terms of transition smoothness and overall running consistancy. Exhaust pressure also allowed the fuel tank to be positioned lower than what is normally recommended due to the slight force feeding characteristics.