I would guess that replacing the stock H pipe ( cats) with either a offroad X pipe or one with cats should be next. With the stock heads the intake and cam are ususally better off stock. Upgrading the TB to a 65mm is a good step. Many have claimed not much gain going to a larger MAF. With more mods you may see some gains there, but only around 3-5 hp or so.
The real barrier to significant HP gains is the stock heads. and you are not going to do a head swap for even close to $600. Maybe you could get the stockers ported and reworked for that price?!?!?
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