Exit speed is an interesting topic. Some coaches base their whole hitting evaluation of a player on it and it's quite ridiculous. Obviously, it's somewhat important because bat speed will allow a hitter to wait longer before starting his swing, thus giving him more time to recognize the pitch and decide to swing or not.
However, bat speed will never compensate for sound pitch recognition, a good approach, and an understanding of what the pitcher and defense is trying to do to you.
In short, I guess what I'm trying to say is high baseball IQ will beat incredible bat speed at a higher level. (provided that the hitter has a minimum of athletic ability; i.e a good enough bat speed to catch up with a fastball)