Custom tune. Run uberdata and tune the car well for premium gas. You'd be surprised how much extra power you can get with a proper dyno and street tune, especially considering that your engine's original fuel map was for 87 octane and not 91 or 93. The chipped program that you're running isn't going to be ideal for your mix of parts or your area's climate anyway.
If you go with internals or forced induction, you'll have to custom tune your car anyway, so you might as well start now.
Oh- you might want to look into an aftermarket cam for your car. They're relatively easy to install, and can net pretty significant power gains depending on how aggressive you go. Zex and Crower make pretty good ones.