Regardless of skill level, optics, distance or movement of gunner or target, vehicle gunners seem to be perfect shots. They don't appear to ever misread a lead. The only limiting factor in their effectiveness seems to be the dispersion of the weapon they're using or straight-up gimbal lock on the turret. For example, Qilin miniguns have a fairly high dispersion, but the rounds appear to average at the aiming point on their target. The Marshall (used in the repro scenario) has a main gun that simply hammers away at targets, even at minimum skill.
I suggest that the skill of an AI gunner affect the speed at which a target can be accurately led.
The pace of adjustment should also reflect whether the target is on ground or in the air. Shooting at a ground target provides feedback in the form of the ground impact location relative to the target. Shooting at an air target provides no such feedback.
The pace of adjustment should also reflect the pace at which the target's bearing and range are changing. A hard-maneuvering target (relative to the shooter) is going to be more difficult to hit than one that stays put (relative to the shooter).