If you're staying NA and have the budget, fork out for the CSL Air Intakes.
The price may make you bend over but the sound it makes is ridiculous.
It was about $6000aud (no including MAP, DME, labour) about 3 years ago when I had my E46 M3 in Australia.
You do need to go from MAF to MAP. Reprogramme your DME. Decided not to go ahead as I have no faith in my mechanics.
HP gain is minimum at best or doesn't translate to laptimes but the music it makes....