CGRetired,
I'm not sure that there is much of a perfomance difference with a Pitts muffler, but there definitely is a difference in the sound that is produced. The pitts mufflers give a lower "grumbly" sound to the engines that sounds soooooo cool!!!!!! I love the sound of an engine with a pitts muffler. The other advantage is that the mufflers are cleaner when mounted in cowls. Most of the time the pitts mufflers can be mounted inside of the cowl with only the two exhast ports sticking out. This gives a cleaner look as opposed to the stock mufflers that come with most engines.

Hope this helps

Ken