Sorry for my english, I'm not trained anymore.
The first gain you have is the Sound of the M3 CSL. This Airbox is based on the one of the CSL. Rumors say it's even thinner, so louder and lighter. The engine gets the max air to breathe. See the air filter here:
http://m3csl.net/de/Technik_Motor.htm To maximize air flow, M got rid of the air-mass sensor in the CSL. So a lot of E46 M3 owners here in Germany reprogram their ECU's to Alpha N enigine management, along with this Airbox, to have the same gains as the CSL. This airbox can be ordered in two different settings, with fittings for an air-mass sensor or without. I don't know the gains with the air mass sensor and without Alpha N. With Alpha N, so without the air-mass sensor, you gain 12-14 real hp. But the best is the sound, you can find examples on youtube. Search for Z4 M CSL. With that Airbox, you drive into the sound, that's much better than any muffler, and it's real engine noise, like GT3s etc. I think the sell it for about 2200€, but I'm not sure. The original Box will cost you about 4500€.
Maybe this is interesting for you, too:
http://www.ca-automotive.co.uk/demo-...3JmlkPTI5.html