I have the GTM axle back exhaust, and it's loud enough to set off warning chirps from other cars in a garage. Based on my judgement alone I wouldn't say it's twice as loud, but it's kinda pushing it at lower revs. It tones down past 3000 rpm though, but I still think it's louder than stock. My sister comments on how she can now hear me coming from the end of the road (about a block) regardless of where she is in the house.
Haven't had any trouble with the law on the streets and highways if that's your concern. Even took my chances and rolled past a couple police officers in a parking lot once. They looked, but didn't seem concerned.
As for mpg, I couldn't tell the difference. Honestly, my average changes every week because of the traffic to work anyway.