In previous GT's, and in particular GT4, racing brakes didn't improve anything, since there was no brake fade simulation, so it was a useless modification.
I wonder if brake upgrades have been completely omitted in GT5 because correct brake dynamics haven't been implemented yet in the physics engine.
As I wrote in another post, brakes are essentially the first thing one should upgrade in real life for serious race track/track day use, as standard brakes will usually fade out very fast and not last more than a few laps on a short sized track.