I'm not noticing much of a difference so far with the auto bright turned off. Perhaps mine was just fast to being with. And frankly I'm a little too lazy to try speed test. Maybe I'll get un-lazy later.
Alright - Test at home
G -- 10 down and .9 up
N -- 8 down and .9 up
Both at full brightness and near the router
But my Macbook Pro was the opposite, faster on N, slower on G.