http://www.tanks-encyclopedia.com/ww2/soviet/soviet_SU-76.php
the ZiS-3 had the best maximum elevation angle of any other comparable
SPGs and could operate to a 17 km (10.5 mi) range.
Short Barelled tanks did not have shorter range.
Most howitzers had short barrels and excellent ranges.
Long barrels gives horizontal hypervelocity FLAT trajectories. Not better ranges.
Well, that is the sort of situation when you have to look at the gamplay more than at historic figures. But still you are a bit wrong about barrel lengths.
1. Tank battles would happen at ranges often much higher than entire maps in coh2 and all vehicles would have to have ranges longer than the map
2. The range you mention is a typical range of an artillery piece. No very useful in tank to tank combat. Basicaly, simplifying it a lot, tank destroyers had long barrels to have high muzzle velocity. High muzzle velocity helped in a) aiming since the trajectory of the projectile was more flat and aiming was much easier b) higher velocity of a projectile meant higher chance to pentrate armour.
So, long barrels were used for a reason and I believe long barreled tanks/TD in COH2 should have longer effective ranges. They were simply more effective in effectively egaging armour at longer distances, especially when the target was moving.