You have a cat-back and high flow cat or test pipe?

Aside from turbo or supercharging nitrous is the best, but that's probably not what you are looking at.

You actually have a lot of different options, try engine tech and tuning forum, you need a specific goal and budget. But you're going to need to invest in some dyno time and engine management to go much beyond bolt ons aside from maybe putting some mild cams and new valve springs in there, maybe an ITR intake manifold too, I'm not sure how much better it is. I'd also make sure your injectors are clean, one of mine was gummed up probably from being so old.