I used oven cleaner one time in the past [the foaming heavy duty kind] on really bad pipes on my HSV Clubsport back home. I was very skeptical of this route - as you can imagine - but it was recomended to me by a detail shop that specialized in prestige sports cars.

Sprayed and soaked them well, threw towels under each pipe, left them over night and wiped them clean with a terry towel in the morning.

Amazing!!!

I've yet to do them since because I haven't found ut neccessary... but will without hesitation should the need arise.

Good luck.