A stall and some nitrous would be the quickest most noticeable improvement

 

You have decent factory heads, you could send them to AI and have them do the magic to them, it's around $1000 if I remember correctly. A smaller converter would help slot as well, I thought you had a different converter until I re-read it. Can always do a tbss intake in there as well to benefit your heads better than your factory Intake. That would just about chew up your budget, may be able to add a stall in there once you sell yours for a new one.