Is it possible to trim the exhaust tubes of a Pitts muffler? Will if affect performance at all? I don't need to do it but it would be nice so I can slide the cowling on my Cap232 easily. I only need like 2-3 mm off.

I think that that would not cause any problem. Especially if it is just for a couple of millimeters. It might increase noise and it might improve performance if you cut it more, but so little will make no noticable difference

The only possible downside that you might run in to is a reduction in backpressure. If the pressure in the muffler falls too low, it won't pressureize the tank very much. On some engines, that can cause the engine to run lean in the air, espeically when the plane is nose-up.

If you notice a problem with the engine leaning after you trim the pipes, the solution is easy. Just use some JB weld to block or partly block one of the pipes. That will restore the backpressure.

So, I'd say go ahead and cut the pipes.

Oh, one last note, if the pipes don't get outside the cowl, your engine compartment will become an oily mess. Not a big deal, just messy. Just make extra sure everything is sealed against oil soaking.