OK guys I have searched and have only learned a little bit. I want to know how these two perform when compared. I know the SR40 will have more lag, but how much. Also, i think that the WRX redline is to early for the SR40 thus depriving it of its full potential. I think that the SR30 with a 17.5 housing would be best because i will have power when i want it and if i was to get the 15 housing it would start to slack as i got into higher RPM's. So do you guys agree with my choice of a SR30 with a 17.5 housing? I can't support my logic, but i am hoping you guys can if you agree with me or if not prove me wrong. Well, i hope to hear from you guys soon. THanks alot. -Matt