We've put around 10 stock cars on a race calibrated dyno in 2013 and 2014. They were all around 200 HP (196 - 204).

We also had some cars with slight mods. Best one was 206 HP.

Exhaust mods in general had a little less max. power.

I know a N/A car that has real 250 HP at the rear wheels. But that had very cost intensive modiications. Ported heads, other camshafts, 200 cell cats, complete different air intake system, new injectors and so on and was optimized on the dyno.

Air filters, ECU tunes, sport exhausts are just a waste of time and money. If you get 10 hp out iof that stuff your'e lucky.

My car stays stock, because i don't care if i have 10-20 hp more or not.

If you want more hp, just bay a SRT6.