Given that the downpipes on turbo cars are really just a piece of pipe with a couple bends in them, wouldn't it be cost effective to go to a good custom exhaust shop with a mandrel bender and have them fabricate something? High flow cats can be had for next to nothing:
http://www.mandrelbendingsolutions.c...,-High,/Detail
A downpipe should be a few hundred at most plus install time. What am I missing - why are these being sold for up to 3k for the F10 M5?