Was recently in an accident where the driver.

Was recently in an accident where the driver rear ended me at about 35mph and I was stopped in stop and go traffic. His car was totaled but surprisingly very little damage to the Aztek. The replaced the bumper and the rear impact absorber, all looks new. Problem is the muffler. The replaced from the cat back with an aftermarket muffler which is what insurance will pay not oem. This muffler is deep sounding, throaty. Also when accelerating hard it sounds like air being forced from the pipe (hard to explain noise). Also I've noticed rpm changes not sure if related. Did idle at 750 in park 1000 in gear, now with new muffler it's about 550 in park and 750 in gear. Also noticed it teeters in and out of overdrive. If in overdrive and you let off the rpm will shoot up a hair then down. I took it to a muffler shop for second opinion, and the guy there thinks I have a defective baffle in the new muffler, possible debris after tapping between cat and muffler, and maybe a sensor problem. His first suspicion was the flex tube as he called it after the intake but he said that looked fine. Auto dealership says they will swap the muffler but don't seem very excited about it. Who do I believe