I was rereading Shibuya for something and actually stumble upon this:
So you flipped the fact for the benefit of your argument. My bad for not checking this right of the gate.
Nothing implies here that 3 fingers Sukuna would still have beaten Mahoraga but completely different thing.
If it was Sukuna from the first time he got to know Fushiguro and his potential he might've lost to Mahoraga back then and there.
So your idea of 15 Fingers Sukuna being at his peak but only with the time limit is actually false.