Check out this old thread. This is with ss3 and prc heads and pretty much everything else you can get done. I bet the ported heads are a bigger difference than going from ss3 to ss4 but not 100% on that but heads are definitely worth 20 rwhp just with ported stock castings. Aftermarket heads I assume will get you more especially with a bigger cam.
https://www.camaro5.com/forums/showthread.php?t=346704