You shouldn't be seeing an increase in "exhaust pressure," supercharged or not.

It sounds like your exhaust is bouncing around, fracturing the weld where your tubing meets the most rigid part (exhaust manifold).
I'd look for missing/broken exhaust hangers, then check for alignment issues that were incurred during installation of your PV muffler.
If all elese fails, remove the entire system and bolt it back on in once piece, from front to back.
A flex pipe or two may solve your troubles if that (reinstall) doesn't work.