It's kinda different for everyone... I'm using the behind car cam all the time since GT2 and it's the best option for me (fastest times ). I'm getting about same times (maybe a bit slower) with... erm, i don't know how to call it, 1st person view? Where you can see only road, but no car's hood.
With every other view i'm uber slow, worst would be a cockpit view, i absolutely can't drive a car with that view. And yeah, i do have a car in the real life (for the people who claims that only the people who don't drive a car in the real life can't use it).
But, for example, of my friends just can't use 3rd person view at all, he can't turn, using brakes too early or too late and so on.
So i suppose you may try to drive a some car on your "best track" a few times with every view and check which view gets you the best time.
P.S.
I just drive it with 3rd person view cam and don't really care if i'm slower than someone, i won't force myself to use a view that i don't like, just to be a 1sec faster... Who cares, it's just a game.