I just put the Flowmaster 40 on my '07 and really like the sound, not very deep, but throaty sound. I haven't noticed any increase in mpg nor more power, but these days fuel economy is a little bit more necessary than power on a 70 mile round trip daily commute, so hopefully I'll start noticing some difference soon.
One thing (if you go with muffler replacent as opposed to a cat-back system)is to make sure the hangars (you'll need extras)keep the pipe well off of the axel! I had the installer redo mine and it is so close to the axel, I hear the rattle over the sound of the exhaust!:angry: