You know your muffler is too restrictive if:
1. The engine overheats with it on, but does not overheat when removed. Better cowling can offset some of this, but if the muffler was not originally designed for the engine, changes to the cowl may not help. Imagining that making the cowling air exit hole at least 17 times the size of the inlet would be an inspired feel-good move here, but it won't do much to reduce the fried egg drama.
2. There is too much power reduction to do the job. If the engine without the muffler--but with the right prop--will fly the plane nicely, but with the muffler and an optimum prop (may not be the same one) will not, then there is too much restriction. Chuck the engine and the non-standard engine into the junk barrel and borrow a "big block" from Proparc. They are quiet, and have enough mojo to pull the engine bearers right thru the firewall.
3. The pressure tap provides so much pressure that even though you run the needle in, practically to the seat, it still runs rich. Or it blows the fuel line off the fittings. Or it blows your Suvvillan super-clunk tank out at the seam. Or, finally proves that you did have a solder pinhole on that metal jobbie, and that's why the uniflow never worked.
4. You can't stand the strained, moaning noise it makes. Kind of an uh, uhhh, uhhhh sound. Like a dead animal trying to roll over and get it over with.
5. It is so restrictive that you can't hear it. At that point, you might as well go electric and keep the oil off your beautiful 20-pointer. But then, you can still glue the muffler on the side of the cowl so you get big boy points. While you are at it, spill some castor on the nose and let everyone admire the front end for half an hour while you clean up after your IC flying session. Go with the rituals. Impression points are real....
Divot McSlow