True story:
I just started autocrossing this year, and thought I was getting to be a pretty good driver. I took my car up to Road America for a Track day. I ran in a couple of sets and thought I did ok, for a nearly stock Focus. I know one of the instructors there and he asked if he could drive the car, to show me some technique. That's when I learned that I wasn't pushing this thing anywhere close to its potential.
His analysis of the car was that it was an absolute blast to drive, but WAY under-powered. He said he had the thing floored through almost the entire course. He said the suspension felt like a go-cart and the brakes were strong enough for the mass of the car. Keep in mind, this is stock suspension and brakes.
My point is.. the limiting factor on this car is not braking, it's acceleration. Therefore, unless I got a stage 2 turbo, I would rather have 15's than bigger brakes. If I had 250hp+ it would be different.
When I replace my 15's with my 17's in the spring, I feel the weight, definitely. I wish I would have gotten the Rotas.
And Formula 1 is a better representation of a no-compromises race car. My understanding is that rotor materials are limited in the WRC rules, that's why they have to use the bigger size. I'm not an expert though..