So then rifling would be required for best performance as current practices suggest and the question is very hypothetical.
Certainly, there is no doubt that round balls benefit from rifled barrels, as empirical evidence and experience shows. So you're correct - my question is absolutely hypothetical. Specifically, whether there's a physical phenomena that explains the rifling benefit for a perfectly round and smooth projectile, or if the benefits only exist when the projectile *isn't* perfectly round and smooth.
I can give you the physical/scientific reasons why spinning a *conical* bullet stabilizes it and helps prevent it from pitching and yawing, but unlike a conical projectile, if a spherical bullet pitches or yaws, this doesn't change its frontal shape, frontal area, center of mass or center of pressure. You can't just handwave it away saying that imparting a spin stabilizes the bullet, because that doesn't explain *how* it actually stabilizes it. The arguments and conventional wisdom that work for conical bullets don't necessarily work for spherical bullets.
And the spinning doesn't negate the Magnus effect - it *causes* the Magnus effect. With a spinning bullet and a crosswind, the Magnus effect causes the bullet to fly high or low, depending on the direction of spin and the direction of the crosswind. If you were to remove the spin (or the crosswind), the Magnus effect would disappear completely.