I've been using 'reduced restriction' intakes and enhanced ECU software (from Autothority) since the mid-80's on various 3ers I've owned ('87 325is, '96 328is, '02 330Ci). They have all improved throttle response with a slight, but noticeable, increase in HP and slightly improved mpg if you aren't driving too aggressively (riiiiiight . . . ), e.g. steady state cruise.

The trade-off has been increased intake 'honk', but since I'm into cars whose middle name is "Motor", I kinda like hearing the motor work out, though I have avoided noisy, boomy, droning aftermarket exhausts. I like the Dinan stuff because it works well together. But it is pricey.

The Dinan CAI is fairly quiet (fully enclosed airbox vs heat shield on AFE) and does provide some ram effect at highway speeds. The Dinan exhaust has a nice throaty burble around town, but is transparent at highway speeds. I also have the "S" pkg on our '04 330CiC Step (the Xer is a 6 MT). The most noticeable improvement on the 'vert came from the front strut brace, which really quieted the cowl shake (not that there was that much to begin with -) and sharpened the turn-in.

We're at 7000' in Santa Fe, so any improvement in HP short of forced induction is appreciated. FWIW, YMMV.

Cheers, M2