1) Pod filter instead of the standard airbox with panel filter, this will allow your car to suck more air ito the cylinders. Be careful when buying air filters as you want to go high quality, K&N are a very reputable air filter brand, and all the well known car mod brands also, HKS, Greddy etc.
2) Make or buy a cold air box to seperate the hot air in your engine bay from the air sucked into your engine. This will make the air colder and more dense meaning more air can fit into the cylinders.
3) Octane booster in the fuel will only give you a short term hp boost with one tank of gas, basically making your fuel more reactive
4) save the money till you can do somthing bigger but more effective eg slapping a nice big turbo onto it, that will make it much moe power
Just a few ieas.