the reason I brought-up those heli tuned mufflers is, they are made/tuned to work at a given band and RPM,, enhance the word "tuned",, if they can be made/tuned to work at a high RPM for a heli then they can be made/tuned to work with a wider band for planes,,

now, if one was made to work on a plane I can understand how they may be too large to fit under some cowls..