Nice Thread Gabe...Personally I don't record audio via my gopro as it sucks...I normally record via my Zoom H1 and then sync in post-production using my car horn as a reference point.
Honestly though under the hood with these turbo 1series sounds wimpy compared to an M3...here's a sound file I made on my blue BMW 135 with it's N55. Recorded on a Zoom H4N (pro audio recorder) inside the engine compartment.
I found that putting the mike in the back of the car (battery compartment) gives a better deeper rumble without wind noise. If you ever put a mike outside the car, make sure you gaffer tape some foam around it and tape it securely to the car.