You mentioned that HG only lets you use a single spinal mount, and anything built bigger than that is a waste. That's one of the reasons I prefer TNE's design system. No matter how big or small my ship is, I can mate a spinal mount to it. A 10MT ship is going to have a spinal mount that is equivalent in size. You can make one that takes up the whole ship, and it's mostly a target. You can make a small one, and the ship is mostly a large missile or laser battery. Or you can make it the "right" size and have a well-rounded design.
In any case, the performance of the weapon is mostly going to depend on the size of the gun, and the size of the gun is limited only by the size of your ship (and how much power you can generate).
An APAWS would be designed the same way a PAWS is designed, although I suspect there is going to need to be an antimatter factory on board to generate what you want to shoot. I suspect that the factory will have a given output rate and capacity, and that this boils down to how long you can use your gun in combat. Once you've used up the antimatter, you simply switch to being a regular PAWS until your factory makes you some more antimatter. (Maybe a couple turns?)
The real question is what the range performance would be. I would guess it would be absolutely attrocious, being jealous of spit-guns.
Just because you fire antimatter at something doesn't mean the target automatically blows up. You do an amount of damage to it based on the energy of your shot. A 1Gj PAWS and a 1 Gj APAWS will do about the same amount of damage, but the APAWS will probably get a better armor penetration value, in exchange for a worse attentuation value (or a worse ideal range, however you want to call it).
Using it on a planet would be fairly useless. You'd probably generate a lot of radiation, and since you need to be close, you'd probably do more damage to yourself than to the planet. There would be NO giant craters or earth-shattering kabooms. Your ship, on the other hand, might get irradiated, and maybe there'd be fallout on the world too. Probably cost YOU more than it cost them.
IIRC, antimatter was usable for warheads under MT TL16, so making AM available at TL15 as spinal weapons is hardly unrealistic.
Side-mounting a fixed mount (such as the side-spinal mount suggested) is probably not a workable design. Your ship is going to be twisting and turning, trying to avoid being shot. Even pointing your nose at your target for a few seconds every combat round is dangerous. It's hard enough to align the nose on a target, but to coordinate with non-nose (or tail) mounted weapons is probably a tactical nightmare! The gun is usable only in an anti-boarder situation, and by that time, you have no power for it.