Grip affects the "at plate" velocity too. There is a boundry layer of air that is trapped against the ball by the seams that decreases the drag. A two-seamer has less seam material exposed to the axis of rotation so it's more affected by drag.
Figure 7-8 MPH slower at the plate for a 4-seam FB, 10-12 MPH slower at the plate for a 2-seamer.
Some pitchers generate a higher rate of spin too. This will help in reducing some of the effects of drag, giving them a FB with a bit less degragation of velocity and more movement.
I'd love to see someone 90 out of hand and 60 at the plate.... it would probably have to be a 90 MPH knuckle ball. That pitch has the most drag and slows down the fastest ... but no reason to let reality interfere with fun.