Personally I use a mod of my own design that rolls back some changes done to the siege units(changes made to the entire game under pressure from certain cheesy MP setups), and even then the Magonels only make, like, 50 kills in battles where the rest of my units make tons and tons.
However, they slowly creep up in experience, and then when they hit about 5 levels of experience... Holy

. Suddenly they are unstoppable killing machines, racking up in the 300s. Why? Hmm, well, I guess a little accuracy bonus from those levels just goes a very long way when you're lobbing a projectile that only needs to land in the general vicinity.(I was testing with Hojo though, so... yeah)