Guys, on an NA car... this isn't a mod for HP, in my experience.
It's for sound and looks. The picture (photoshop?) shows the Remus exhaust getting rid of an M3 design "flaw" -- that ugly muffler hanging down in the rear.
Combine that with a perhaps nicer sound than stock, and there's your $1200 benefit.
Also as Greg said, what does it weigh? Some stock systems are VERY heavy, as much as 50 lbs heavier than aftermarket, and that's where you'd see a performance gain....
Looking forward to a sound clip, possible side by side with stock, in car and out!