RE: Back Pressure?
With a general type "muffler" Pitts or can with down tube, it's usually more about tuning. There's a small difference in back pressure but not as much as one might thing. It's probably more about refelction, or when that back pressure bounces back into the cylinder. part of the intakeexhaust charge timing cycle. Where is becomes a lot more apparent is when you use pipes and the header length is wrong. You can end up with an engine that's hard to start or runs badly because of back pressure reflection,
For what you have focus more on re-tuning. You may still end up down a little bit, that's common with Pitts style mufflers. Pinching tips or reducing exit size won't get all that's lost back.