I looked at QBs making a championship game or maybe it was actually SuperBowls. QBs taken in the first round were more likely to get to that level then QBs taken eslewhere, whent taking into account the number of players taken. And it was even more dramatic for top 5 picks. I forget the span of years I looked at.
But when you take into the account the number of players picked, I think your chances are much better with a 1st round pick.
Now with that said, how many 4th/5th round picks is Griffin equivalent too? If you tell me I can take Griffin or 2 4th round and a 5th round QB and have value left (i.e. picks that I can use to address other positions), then things start to even out I think.
In terms of a single pick, I have no doubt taking the early QB is the better bet. In terms of real value, especially with the new CBA limiting rookie contracts making those picks worth more, I'm not sure that's really true.