i have k&n and been using it till january on the coupe. it sounds a little different than the stock, performance? i dunno maybe a little bit.
I had ECIS CAI before on my e36 m3 and it sounds a little bit wilder than the stock/k&n filter. FOr the power gains, ECIS is one of the best out there. Or ECIS + Jim's filter = good gains too.