I think springs would be a good investment. The 2.5RS is not as fast as a WRX is. It'd take a lot of work to get a lot of power out of it. Definately not for 200 bucks so suspension, IMO, is where this car will shine.
I'm thinking about selling my Borla header if you're bent on power. I'd do less than 200 shipped. Don't know how soon you need it tho.