interesting debate on aftermarket exhausts...I initially put on free flow exhaust and the performance was great, but I lost the 'quietness' inside the cabin...on those days coming home from work, I didn't care too much for the slight drone -- it wasn't bad like I've experienced in my other friends' cars but I'm just picky...don't know how the inside cabin of an M sounds with either the Stillen or Megan set up, but would be curious since everyone only describes the 'rumble' outside of the car...
I ended up taking out the aftermkt muffs and putting the stocks back on w/ bigger tips -- also completely repiping the underside...took out everything from the Cats...used 2 1/4" piping all the way thru (dual from cats to x pipe to highflow resonators to stock mufflers)...what a difference...I have the inside cabin 'Drown Free' plus the performance increase compared to stock piping... I will be looking to visit LSD motorsports (i'm local) to check out the header and spacer set up...then get my car tuned....