After many suspections and observations I finally did a test.
I took a role of commander in various addon tanks.
I had noticed that addon vehicles usually do not fire vehicle main gun at as long distances as BIS vehicles do. And yes it is so. AI gunner can barely hit anything beyond 200m while BIS tanks are dead on target almost always at this range.
So how do AI gunners aim? The strangest part is that if you act yourself as a gunner in an addon tank (using the gunner view to aim normally) the hits are as expected - most of the time kills. Been moving around memory points, but this seems to have little effect.
And the the funniest thing is that AI seems to always aim too low and undershoots.
EDIT: Any ideas of how to "adjust their sights" are most welcome. AI is clearly not using the gunnerview in the same way as a player does.