Was using the blitz filter during my E46 320i days. This filter does gives the extra HP at mid to high end. We(Alan, Shawn and Sam) did a test too using the scan tool tat plugs into the ECU.
Reading real time values from a Palm while driving and blitz gave the higher air flow and cooling down air temp compared to GruppM and stock filters.
Yes, the only set back is tat it doesnt filter much and true enuf one day (after using for about a year) my E46 stall intermitently. My MAF kaput! Tats not all, dismantle my idle valve and throttle body and found so much dirt.
For those using blitz (Sam and Gordon), suggest you do a half yearly cleaning of the MAF, idle valve, throttle body and air manifold. I know its a chore but end of the day its some saving you gain from hefty replacement parts and petrol consumption as efficiency improves as well.
Actually, Im still using the blitz on my evo. Initially tried the Apexi but high end suffers. Then tried the K&N, better but still finds the high end not so eager. Finally settle with blitz, mid to high is superb!
One experience is that the K&N has better low to mid torque as compared to blitz. This can be verified from my boost meter.
With K&N, my boost spike to 1.7bar in a flash but with the blitz, it only spike to 1.5bar.
I still swear by blitz for now.
Cheers!