RE: Muffler types
Each of them has a particular application, the stock muffler is the one that has been proven with the engine and supposed to work best and it is intended to have the best performance of the engine with an acceptable level of noise (Sometime the acceptable level of noise reduce the engine efficiency)
Pitts muffler is intended for an esthetic application and may affect the engine performance but will fit fine in a scale plane and will look better.
Tuned pipes are a wide book to read from. A good tuned pipe must match the engine size and has to have a particular length, and then it will produce a better engine performance. There are other facts that will produce a better engine performance using a correct pipe. Not all pipes work for all engines, as I said it is a wide book to read from but in fact it is the only one kind of muffler that I know that can produce extrapower from any engine.
Alfredo