More than 400hp?? You aren't going to get a measurable amount of additional hp without forced induction. I got intake and exhaust for the sound, did it add power? A little. Do I think I can feel the difference on the street? Not really.
If you just moved to 400Hp and you are already not satisfied, I don't know if you would be happy once you got to 500hp.
The Beast: E39 Dinan M5 (Gone)
Beast 2.0: E92 M3
"Control, Vision, Determination" These are the three fundamental components of the new generation race car driver!! SPEED is a by-product! -Memphis Raines