With no other mods on a 2004+ GTP step one would be: A 3.4" to 3.6" pulley (the smaller the pulley, the more support mods you will need to keep it all together), cheap cone intake and thermo stat.

Those would be a starting point.

Step two: Replace the exhaust (from the headers back if you want to spend $$$), and a reprogrammed PCM.

The big diff to keep in mind is the Cobalts and Ions come factory intercooled where the W body platform supercharged cars do not.

GM cheaped out and uses PCM reprogramming to keep knock down - which primarily = running the motor rich and dumping large amounts of fuel to stop ping / knock.

Step three: Rockers (or save money on these and wait for cam in step four). Cheap Intercooler.

And last but not least Step four: Would be internal engine work (a cam install + all the necessary porting and polishing) or fancy intercooler would be last.