Hadn't even thought of that angle, but conservation of energy would apply(?) and Bernoulli may even have something to say about this aspect.Going from Zonie's calculation of 350 rotations per second, a 50 caliber ball would have a surface velocity relative to the air of 45.8 feet per second. That is, rotationally, or perpendicular to the direction of travel. Not much compared to 1000-1800 fps forward. (note: 45.8 fps at the "equator" of the ball, going to zero at the "poles.")
Still, with really long range shooting the shooters have to account for lift or drop due to crosswind because of the Magnus effect. If the bullet is moving up or down because of the crosswind then energy is transferring from rotation to lift and the rotation is slowing.