Who actually uses siege frigates? I know the AI does raids of 10 of them, and it is scary. But 3 hangar defenses with fighters can gut any serious Siege frigate raid. They take 12 supply. 3 of them take nearly the supply of a Dreadnought. A Dreadnought provides vastly more ship to ship firepower and comparable bombing power to 3 siege frigates, but the Dreadnought doesn't pop like a balloon when a fighter gets near it. I don't understand all the siege frigate whining. Counter them with fighters.
My suggested buff - keep the costs, health, and damage the same. Bring down the supply to 8. That way I might consider building a few and bringing them along instead of teching up and buying a dreadnought. As it stands, I don't build siege frigates because they die too easily and take way too much supply. You cannot mix in 2 or 3 without losing a real combat edge. Not to mention if you only bring 3, they will be focus fired by a competent opponent.
How to kill siege frigates:
Hold Alt.
Left Click a fighter squad.
Hold shift.
Right click each siege frigate individually.
They will die before they kill your planet IF you bought 1 emergency fortification upgrade.