I think they should be, at least, equal to the unit of the strongest unit (not cap) of a strongest starship.
Why comparing to a starship unit and not a starship cap? In this case I think it's moot because the strongest unit starships are the 1-cap ones (iirc, at least it'd probably be wrong otherwise) but I'd think you'd want to balance vs a cap of the stuff where cap is more important than individual unit costs.
Anyway, when you say "at least equal", do you mean equal? Or more like 2x as strong? or 1.5x? or 4x?
I don't unlock sieges because of their hull bonuses. I unlock them because they are brawlers. Is it so bad to just make them brawlers, with no hull bonuses but a high base dps?
Well, you don't unlock them at all, right? In terms of K, I mean.
Making these just straight dps with no bonuses is fine, was mainly going off the name "siege tower" to infer that they should be good against fixed/heavy stuff as opposed to just all around mauling whatever.