The question was, in fact, relating to a "certain new vehicle." :) The stock and optional wheels on the STi are both 18x8 (or possibly 18x8.5...there's some conflicting information floating about)...the BBS wheels you pay $2k more for are 7 lbs. lighter per wheel. My thinking was that wasn't worthwhile even if they gave me BOTH sets of wheels.
According to Tire Rack specs, the 245-45-17 and 245-40-18 are the same outer diameter, the same weight, and $70 less *per tire* in the Bridgestone RE-01R. So I think my plan is to use the stock 18s for street tires, and see if 17x8.5 Rotas will clear the brakes. Thanks all for the advice!