I have Turner Motosports "Test Pipes" (Primary cat delete pipes) connected to an OEM section 2 exhaust with the secondary cats still in place. I also have the Delete-R so you will not get O2 inefficiency codes with this mod. This package alone produces 20whp, with a chip it's 40whp.
Why buy test pipes connected to OEM section 2 instead of modifying your own? Well it'll be pretty hard to sell your car with an illegal mod, purchasing replacement from BMW will be very expensive, and because it's only a few bolts, if you have this and keep your OEM you can go back to stock in about an hour if you need to go to the dealer. No warranty issues
This is the best bang-for-the-buck mod. Shoot me some offers.