Maybe I misinterpreted this and some of the other scenes/statements, but the extra set of arms and mouth giving Sukuna the ability to utilize chants and hand signs w/out it hindering the rest of his body sounds like an increase in efficiency and/or output.
I doubt it's a "hard rule" that the # of limbs has an absolute effect on your Jujutsu but more like a case by case basis. For Hana and Naobito, I believe there under performance after losing an arm is more off them haven't recovered from the injury and/or learned how to compensate for lack of said limb (Especially Naobito).