How to gain more HP in my 2003 Pontiac Sunfire..?

[Mopar 4Ever!]

I have a 2003 Pontiac Sunfire 2.2l ecotec. It has 80,000mi on it and comes stock with 140hp and 150lb torque. I want to up the HP in it, I just ordered a K&N cold air intake, they claim it adds 8-10hp for my engine but I don't know if I believe that. Are they any other ideas for HP gain for this vehicle? Thanks alot..

 

[melsg5]

Cat back exhaust system and headers.

 

[proudgtp&gxpowner]

i had a 04 sunfire, i just put in a k&n air filter and put on a dual magnaflow exhaust system and ran synthetic oil in it and it ran 16.30s