I haven't really used cats much because 1.) they're really slow on land, 2.) they take a ton of pop that's not proportional to their usefulness, and 3.) they don't really do much to a wall if the city is stacked. In fact they don't really do anything. The point of a catapult is solely to demolish walls so that foot troops can move in. Technically, they would have longer range than any archers so stacking shouldn't be as much of a factor in wall demolition. Although the idea to make them hurt buildings too might be worth looking into.
If any implementations are made, there might need to be multiple tries before the numbers work themselves out