That's very 'high horse' for you, Zin!! Unexpected!
I'm extrapolating the data from constant forms of fighting and movement to an lessened discipline and thinking it's not too far a jump in fitness. I don't think it's an unfair educated guess. If you take it the other way, are you assuming a boxer could do all the forms in MMA at a constant level for 25 mins? Cos that's what you're saying!
And, btw, I'm not equating a 'one off punch' to anything, I'm using the data Mayweather, himself, offers; the temptation to stand and trade once gets comfy. Who knows at what point that will be, in the fight? But, he DOES do it! His 'rope-a-dope' is the only thing that saves him and he likes to do it.
I remember Shane Moseley catching him and it wasn't at distance, IIRC, they were close quarters. That shows Mayweather takes risks.
I wouldn't count out McG due to Mayweather's own ego.