Okay, let's consider the 25% damage boost.
In order to take advantage of it, you have to get your fleet to the buff.
So you can use it defensively, which the other superweapons aren't so good at. On the other hand, if your other defensive advantages (SBs, tac structures, culture) weren't enough, how did you manage to afford a superweapon?
You may be able to do a deep strike with a fleet, but that's painfully slow unless you're Vasari. And if you go past one SB there goes all your AM.
So that leaves the outer worlds of another empire. Unless of course you're steamrolling someone already, of course. So you drop your Signal, move your fleet in. Opposing units lose their culture advantage (AM for TEC, +DMG% for Vas) and you gain mitigation + 25% damage. So far so good. Now your opponent can
- Defend
- Counter elsewhere
- Do nothing
I think we can agree that "Do nothing" is a bad idea, so let's forget that.
If they defend, you're at 125%, plus culture boosts. Could be tough. But if they spend 16k creds + resources on ships (what you spent on super-weapon research and construction) they'll probably be ok since that's enough for an additional 25-32 or so cruisers or 40-58 LRF, depending on race. The 25% boost does start to favor your assault if you've got 1000+ supply (~50% upkeep) in the well. Under 1000 it roughly favors the defender, but that doesn't include the mitigation boost (including that is complicated thanks to Vasari tech).
If they counter elsewhere, you get to complete your assault in 80% of the time it would have normally taken (yes, 80%, not 75%). Or with 80% of the forces in the same amount of time. I don't see either of those 20%s making a huge difference in your ability to respond to their attack.
It still doesn't feel very compelling to me. It relies on you leveraging the size of your fleet, which isn't something Advent is good at. You don't have very strong econ (especially in the resources sector) and you lack the fast fleet building of other races in the late game (RA and IJ+DM+Econ) to really explode into large fleet territory. You have to be good at something you're racially bad at to get the most out of it as an OFFENSIVE weapon.