One of the problems with wheels/tires and the Spec suspension is interference between the tire and the spring - especially on the 2005 and later cars. Toyo makes a 225 R888, but it rubs. Going wider and taller with an 18 inch wheel is llikely to exacerbate this - and cause a lot of guys who are already running to have to dump 8-12 wheels each
PLus - the SVT wheels can be found from the sponsors on this board for as little as $104. That's basically free as far as racing goes.