Just wanted to take a moment and introduce myself as I’ve been perusing your board for a bit. You guys have a great setup and a lot of good information. My 6 is the V6 version, which shares the 3.0 Duratec block with you guys. While I love the power of the car, especially with a 5 speed manual, I have found that the power curve is kinda rough and falls off in the high end. I’ve got a bit of a tuning dilemma b/c the U.S. is the only benefactor of the V6 in the world, so all the good tuner parts are going to be for the I-4. I also don’t want to violate emissions rules, so I’m not looking to do anything to mess with the Cats. I’m looking at getting a custom mandrel exhaust system built, to help smooth the power band out, what have your experiences been with exhaust mods sans headers? Which mufflers do you guys find to be quietest? Any thoughts would be appreciated. Thanks for the insight.
-Alan
I think 2.5" mandrel bends will suit your 6 just fine, and any straight through muffler will produce the most power. Magnaflow has good sound (deep) and Dynomax is supposed to sound good, too. 2.5" systems net ~8 peak hp. (at the wheels) on our 2.5L engines, so expect something comparable. Again, welcome! 
There was a post in the dyno section of a guy taking a Mazda 6 and adding headers and custom y-pipe and going from 167 hp to 182, go and look for that for the details 