If the boost isnt enough to it could also be more. I just looked at the stronger mons with gender like dragonite, urshifus, landos or like you mentioned Darm-G and thought that buffing them by more than that would make them broken. Even a mon like conkeldurr would have the equivalent of 187 base attack before items if its adamamt and fully ev invested in attack, more than any ubers mon atm. But if nevertheless only few non ubers pokemon become viable, the boost could also be bigger.Also, it feels weird that you are considering an ubers-based metagame for this. A flat 25% boost to power, accompanied by a 25% boost in bulk means that the meta doesn't seem much more offensive or bulky overall, so it's not like the overall power level is inherently increased to ubers levels. Most gendered mons are still going to be sub-ubers level in power, I think.