Interesting, my 200 yard accuracy load for my 14 bore muzzleloader, shooting a .684", 480gr. ball @ 1,400fps, becomes subsonic at about 80 yards. This rifle shoots 1 to 1.4MOA at 100 yards and shoots under 2 MOA at 200 yards - both off a single bag beneath my left hand.

Interesting that the sound barrier means little to this rifle's round balls.

Dropping through the sound barrier is more of an accuracy barrier to long range greatly elongated projectiles, such as match bullets fired from .224's through 8MM's in 1,000 nad father competition shooting.
The very large bullets shot in bore guns are very much less effected, it seems. But then, this is a suposition by me - perhaps the slugs are effected as you suggest.

I know for a fact that properly stabilized round balls aren't.