Alright, siege engines on 5.080.
It looks like a full cap of Mk I and Mk II siege engines do 5,931,250 damage in a single salvo against a target they have their bonus against. (A force field in this case).
The AOE also killed everything under the shield in that single salvo, although it is Mk I system and their was not much underneath it.
So, not totally worthless but that damage drops down to 1,186,250 against targets without their bonus, that's 2 to 4 salvos to kill a Mk III guardpost assuming I lose no siege engines to the AI's response.
In other words, still not sure. We'll see after the numbers fix next patch gives them a bit of a buff.
D.