I just looked up the BMW performance intake and
wow expensive. Definitely not worth the $$ for such minor HP gains, IMO. Pretty much any aftermarket intake will be fine. I had a simple K&N cone filter on my previous e36 328i, cost me $50 or so.
If you're that worried about it, just call your dealership and ask if aftermarket intake would be OK. I doubt the majority of places would even mention it if found during maintenance, but attitudes regarding mods/warranties do seem to vary from place-to-place. You could always just drop a high-flow K&N filter into your stock box. Or, install the aftermarket intake without asking and take 10 minutes to switch back to the stock airbox before dealer visits (honestly would be my choice
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