Most of the 2.2 Owners here do the Following:
De-Restrict the Air Box(I done that)
Aftermarket Air Filter(K&N Panel for me & Aus_SRi)
Change the Exhaust to 2"-2.25" all the way Thru(I have 2" all way thru,Stock Rear Muffler)
I also have a Chip,Exhaust Cam and Extractors that came with the Car(2nd Hand Demo)
Doing the 1st 3 makes a Difference, Better Acceleration and so Forth.
I find mine has more Torque and has Power right up to Redline(6500rpm).