Quote Originally Posted by Grond
I'm kind of fond of the sniper catapult (with Baush & Lomb optics), myself. As they get up in valor, they seem to be able to drop a rock right on top of the enemy general. I'm not sure a catapult built in, say, an iron rich province will get a bonus with using their weapon (it's not like they're iron-shod rocks or anything). And... it's a rock. It either hits or it doesn't. So this probably doesn't answer the question very well, and I'll withdraw my answer.
Siege weapon accuracy is bugged, but I don't recall right now what the problem was. They get an accuracy bonus under some circumstances, for example when targetting a general. But they don't get a bonus from weapon upgrades. Weapon upgrades only influence melee attack (same for all missile units).