I've thought about the 'parachute' effect when the oem muffler was replaced with a smaller aftermarket muffler, and how you could vent the rear bumper cover to avoid it. also thought about adding something like this that would run from the transmission pan to the rear bumper cover (trim the cover so that the diffuser came just under the licence plate) - one on each side of the exhaust outlet:
Universal Rear Diffuser Kit Under Body Spoiler Increased Down Force Reduce Drag | eBay