I don't think the availability of siege ammo is all that relevant. Though I might lower the ammo counts a bit.
The bigger issues are the incredible ease with which even small numbers of ships can reach the damage cap (like, call me crazy, but I feel like maybe if all the attackers can muster is two light gunboats, they shouldn't be able to hit the cap), and the incredibly slow repair rate.
What's wrong with gunboats? Would it be better if it was two battleships instead? The manpower invested is the same in either case.
