I've heard that just replacing the stock manifolds with for example hooker headers is good for usually 30hp?? This true?!

 

probably on a high output engine, but not on ours. You will see a good gain though, definately worth the money.

 

headers+catback exhuast+highflowcat=25-30hp

 

dont run a cat, and you will prolly not see those gains on an otherwise stock LO3

 

Yes headers on a LO3 are worth a lot of hp. I picked up 5 mph and .6 sec with headers and a 3" cat.

 

If you were to change your headers and exhaust on your lo3 over to 3inch everything you will see a better gain than on the high output engines with the same exhaust setup, since the low outputs got the crappy exhaust manifolds and 2 1/4 exhaust.