Start with this,
It will add about 10hp and running 87, and you can get higher with premium.
you have an intake and exhaust.
You can ditch the throttle body spacer, I don't think those have ever been shown to add HP.
After that there are e-fans, better plugs and wires, coil packs, underdrive pulleys, and last but not least the Explorer Express Supercharger that rings in at $4k.