while trying to manually shoot the scorcher at low angle i tried to use the output provided when shooting with the artillery computer. but i noticed that when i shot repeatedly at a target, and switched everytime back to the gunner view, the gun elevation would never be the same. i did a little test, shot 7 rounds on a target 16,3 km away, angle ranged from 78,28 to 78,96 which results in a spread of 936m (if my math is correct).
i also did a test with the same range for the sandstorm, with similar effects: angles ranged from 77,54 to 78,19.
i don't know exactly how big the dispersion would be without this effect, but from my experiences of guesstimating the low angle manually it would be around 10-50 m, depending on the distance and type of artillery charged used.