Okay I have an 02 mazda miata special edition , my few upgrades include a harddog m2 hardcore rollbar, some cheap ass muffler from ebay(sounds good tho), and a K+N Air Filter, i mean my car sounds good and is fairly quick until 6th gear, but i don't know what else to do to it. I have a really short spending limit and i don't know what i can do to up my horsepower , i'm estimating i have about 147 HP right now altogether. but i need some help on what to do next.

any comments and advice would be greatly appreciated.