I'll look into the DLC, although it does seem to me like one of those games where the 'additional content' is stuff that should have been there in the first place. I know it doesn't cost much, but it's a bit bloody cheeky seeing as there aren't exactly a ton of tracks and cars in the game.
Anyway, I'm still trying to get all the medals at the moment. I'll do that whilst trying to beat times on earlier tracks. Perhaps the DLC stuff can wait a week or two.
(As well as being signed in to PSN, you have to press square on the top level menu each time btw, to get leaderboard stuff working right. I quite often forget this!)
There's deffo confusion on the leaderboards about versions, or at least times updating more rapidly on one system than the other on the ps3 leaderboard it has and older time than on vita.
It kindly gave me a second platinum when I logged in though