I added a 3" TTI X-pipe and exhaust system to their step headers, which extends to Dynomax Ultra-Flo mufflers (I know - bad word.) in front of the rear end housing, about a year and a half ago. Unfortunately, I wasn't able to do a back-to-back comparison, but from all indications, it did help, especially after I took the 60lbs out elsewhere that the system added! It doesn't cause any major access inconveniences underneath, but it wouldn't have stayed on if it didn't help.
http://www.ttiexhaust.com
It's Mopar-only headers. But the pipes can be adapted otherwise.