get the STi and modify THAT
why play catchup? i love the personal touch of a modified car, but start with the goods, then go from there. if the STi was just some bolt-ons and computer work above a WRX i might say just mod the car you have, but it's a very different beast, if nothing else, it has a completely different, more powerful engine. i think it would take a good bit of work to get the WRX powerplant to put out STi numbers, and you would be closer to its limit, as opposed to having an STi with room to grow.
i agree with politico, the only real competition is the Evo, and i'm a subaru man.