I've been told with these nnbs rigs, though, that headers aren't really necessary because the stocker exhaust manifolds are already pretty good and not very restrictive. Thoughts??
From what I've learned through some research is that the shorty headers won't lend very much to performance gain for the amount of cost incurred. You'll have more beneficial gains with the long headers for the money.
HFCs (High Flow Catalytic converters) in place would most likely help, but I haven't found anyone with them installed to know about the benefits. I have them installed on the BMW and HP gain is right at 17 rwhp along with the cat-back exhaust.