You're assuming that GT5 already takes into account all of those variables rather than something that would need to be calculated in addition to what is already modeled by the physics engine. Nothing about how GT5 plays tells me that brake material (or even brake type) is differentiated by the engine. Maybe there's something I'm missing (I never drive with ABS off, so perhaps that completely changes how the brakes work), but you're acting as if this is all already there and ready to be implemented and I don't see why.
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I'm not assuming any such thing, and your lack of ability to determine whether material plays a role in braking in GT5 is immaterial.
What you do is set up a heat transfer model, with capacitance according to rotor mass (approximated from dimensions, or known), put heat in relative to instantaneous braking duty (mostly dependant on the tyre model) and remove it according to the available cooling duty from the surrounding air. That latter part is the tricky bit (but easily faked, like GT's ABS), the rest is pretty solidly described by theory, taking a few reasonable assumptions.
Brake dimensions and materials are probably more-or-less public knowledge, material properties are easily discovered and everything else is already there in the game.
Then, for the (possible, not confirmed actual) reasons I've already explained, you construct
generic brake pad compounds that mimic the sorts of things you get in real life, and have specific temperature operating ranges and associated friction profiles that return stopping force vs. applied force (pedal position, brake bias, ABS etc.) and temperature (from the heat transfer model). In that sense, they'd work much like the tyres do, and so I bet a functional brake fade model could be copied from parts of the existing tyre model. It'd maybe plug in to the physics engine as a modifier to the braking force numbers coming out of the ABS.
The model, whilst crude as I've described it, would also implicitly enable varying disc sizes, materials, cooling etc. and is agnostic to any kind of brake control systems (like ABS) that might feed it with different brake power numbers. Once the model is working, it's just a case of data entry, like the rest of the physics - tuning of the systems can be ongoing without upsetting that process at all.
Anyway, I hope you didn't miss the part where I said I doubt it'll appear in a GT game.
