I'm suggesting that real GT4 cars would also be implemented as upgrade paths for the relevant base car. If there were multiple cosmetic variations for a single model* then the player could choose which they want.
*because for the purposes of BoP it's probably going to be easiest to make all instances of the same model equal in performance even if they're technically not in real life.
Anything based off modifications of the same platform should be an upgrade path, imaginary or not. I don't much care about the difference between a GT4 that exists in real life and one that is "fictional", provided that it's done well.
And yes, I'm sure Polyphony would mess up the implementation. But since we're already talking about fixing something that they're doing poorly we could fix that too. Upgrade presets to make a car into each spec. Forza already has one-click options to auto-upgrade to the top of a PI class, GT should have that for specific BoP classes like Group 3/4.