Looking at past GT Academys, it helps a lot.
I think almost every winner has had some past experience. And quite a few fast guys in GT5 are fast in real life.
GT Academy is about converting from the game to real life. So any knowledge you have on the flip side is obviously going to run in your favor. It's a little more than just a time trial.
GT knowledge passes onto real life but not vica versa (considering that you know what apex is, talking generally so don't get offended).
I'm fast on GT5 and I have a feeling I'd be the same way in reality too. Hence I plan to go karting sometime soon.
GTA isn't all about GT5 I hope you're aware of that, but real life experience helps with GTA's aspec of real racing. Real life experience won't make you any faster on GT5 but will in real life, that's my point I hope you get me.