Paul Smith says the 85/15s are 58/15s without covers. I have both pickups, though my 85/15 guitar is a CU24, not a McCarty, so that makes the comparison unscientific.
But I'll plow ahead with this apples/oranges comparison anyway...
The 85/15s are brighter and seem to drive an amp a bit harder. So that'd be what I'd expect with uncovered 58/15s.