A proper Galleon is most definitely a kill ship. As for the damage vs hitpoints talk, meh. People take piercing because it has the longest range weapons available with that purpose. People take mortar because it's OP. And that's that really. There's no point in spending brainpower on the issue.
If the pilot wants, he could be running a much more brawly build by using carronade flamer, or he could go longer range and use lumberjack + explosive weapons. Though both of these options are vulnerable to the range of a ship with gatling+mortar on board (effective range vs arming time).
I do however think, and have thought for a while, that the repercussions on killing a support ship is too high. The ships would be balanced around this concept, but not really. And since hull is zapped the first time the armor goes down(by the OP mortar+greased rounds) anything with a low armor value is not really viable. That is why Pyramidions and Junkers are so common and why the Galleon is picked over Spire.
Instead of having it cost one full point to lose a squid or goldfish, it would make much more sense to lose just a fraction of it. Then the hull life on these ships can also be lower and the ships would see a lot more use far more easily(because then they are cheap). In comparison now, a ship that gets lasered out of the air like nobody's business, like the spire is now given the same worth as killing a Galleon that can stay in the air almost indefinitely, doing the exact same thing, just better.
The point system I'd propose is the one used in the Gundam VS games.
http://www.youtube.com/watch?v=CX4qE5FhfjU In the loading screen you can see their points (2500, 3000, 3000, 2000), the max points to be expended in that game is 6000. That is the green and red bar you see in game. When a suit dies, it's points are taken from the amount of points a team has to spend. So if a 3000 ship dies, 3000 points will remain, if it dies again 0 points remain, if either ships dies then they lose. Support suits in that game are far less costly than the mid range and high range suits in that game, which is also why they see quite a bit of use in there.
The same thing could be done in GoIO making it much easier to balance the game and balance certain ships in the competitive context of the game.