You need to be looking at the exhaust side of things more than the air filter. There are offerings from MMC (Stage 1 system), G56 and Krazy Horse at the moment, plus one or two others that involve a bit more fabrication.
+1
I had the MMC stage 1 fitted in 2 stages – first the air filter which made no perceptible difference and I still had pinking at high revs.
Adding the exhausts transformed the car with smoother running, freer revving and no pinking.
