From my tests, they're not terribly powerful, not as good at offensive fortification as defence towers. However, they are able to do damage to rams, if properly placed, which could be their main use.
The existing tower upgrades only apply to defence towers, sentry towers, and the Han Great Tower. It doesn't matter if they're researched, the artillery towers maintain their minimum range.
I actually decreased the range of these towers from the range of the siege catapults. The siege catapults have a range of 40-100 meters, and the towers have a range of 40-80 meters. This seemed logical, since historically it would have been hard to fit a large siege catapult in a tower, and it would be better for gameplay to avoid offensive fortifications. However, an approaching enemy, such as a ram, can pass through the area of fire quickly enough to reach the tower without being hit. I see three options to fix this. First, I could increase the range of the towers, which risks making these more offensive buildings. Second, I could decrease the minimum range to something like 25 meters. Lastly, if others don't think this is a problem, I could leave it with its current values. What does everyone think?