No, stupid idea... The burnout thing was really stupid, everyone paid the money, and it suddenly didn't matter that you put weeks of work into getting all of the gold, or platinum or carbon cars... Everyone and their grandma had them.
Burnout had a tiny fraction of the cars GT5 has, plus it wasn't that much work to unlock them all anyway, the only difficult one was the Uberschall 8 Carbon and even that just meant you had to have two friends and a bit of time. Buying a time saver for GT5 would mean unlocking several times the amount of cars, and would be ridiculous (so I am actually agreeing with you after all...?).
It would be odd for Sony, a company that doesn't charge gamers to play online even though it would be acceptable as a certain other company does, to suddenly let you spend money on unlocking things you'd eventually get anyway. EA can do it because they're third party, Sony, as a first party publisher, has to maintain a certain level of quality and value for money, though I can't really explain why I believe that.
It would just be a really strange move for Sony, at least that's how I see it. I'm sure some people would pay for it, but it would be a somewhat dangerous precedent; for a first party publisher to condone shortcut DLC (a practice I really, really hate, personally), the level of what's acceptable in the world of DLC would be lowered and we'd be one step closer to having to pay extra to finish a game and such.
This is just how I see it, anyway, I didn't mean to present this as fact.
So, essentially, I don't like grinding, but I'd far prefer it to Sony lowering the DLC tone, if you get what I mean. Anyway, there are so many alternatives to grinding that that doesn't even apply any more; Rubber band grinding, automated B-Spec/Remote Racing... Actually, that's it, but even so.
On a final note, I hope PD do include a mode of letting those who can't be bothered to actually play the game they paid £40-50 for get the cars they want in GT6. Why not just have all the cars available in Arcade mode or something (aside from the fact that the list would be massive)?