There might be some slight correlation, but I'm sure many people defy them every year. The MCAT got the best of me once, but I retook it and did alright. Frankly, I found some material (general bio, organic) so uninspiring that it bored me out of studying it with the kind of passion that one needs to score super-high. Out of principle I was willing to make that trade off. I think the med school material will be more in keeping with what gets me interested, so I expect to do well. But it still comes down to motivation, heart, and of course your cerebral capacity.