Regarding per-car leaderboard entries, filtering what already exists by car could totally be done.
But actually creating separate entries per car -- so that you could have a separate record for say a Stratos and an M3 Evo -- would demand a database that's 4.5 times larger.
Doable, but really unlikely they'd consider the positives to outweigh the negatives on that.
Maybe even the storage space isn't really a problem with today's standards of things, but altering the database structure -- especially since this affects what defines a unique key -- is a task that starts to require a good chunk of time and risk that the devs would want to avoid. The required UI changes would then be more work on top of that, as well as design for where and how the new feature would appear in the game.
All totally doable, but the cost and risk factor are way beyond the benefit factor for that one, I'd say.