Not sure where your getting 10min upgrade time for that many upgrades especially since if I remember right not having the t4 engineering suite (engineering or combat version) makes all the level 3 weapon mounts take about 15mins to construct by itself. All upgrade times are standardized between the factions so it should take just as long to max out the ACU reguardless of faction.
Tier 3 defensive (backpack) or offensive (weapon) upgrades are on par with experimantals. So your level 3 gatling was balanced against the beam style weapon systems commonly found on experimentals. That said its really good at taking them out especially since the Gatling/Beam tree is designed for single target kills while the Cannon/Artillary tree is designed for AoE army munching
If you want the hard numbers on the settings used in this mod I could dig up the old balancing spreadsheet... I believe its still posted somewhere within the 60 page thread for the project on the GPG forums