It's one or the other imo. Either you try to imagine a 205 pound Marciano attacking the modern division. Or you imagine the Rocky we know attacking the CW/LHW division. I just don't see the point in picturing a 189 pound man running a HW gauntlet today.
I agree with you that the additional weight affected Spinks. But he was fighting a guy who outweighed him by about 50 pounds. I wouldn't have fancied his chances against Larry weighing 175 though, and I don't think it would have been allowed because of the limit. I think Spinks could carry weight better than Rocky could have done though. Yes, in today's era, Rocky would have been a CW or a LHW, unless he'd have bulked up. But again, in my opinion, a bulked up Rocky wouldn't have been as effective.
But a bulked up Spinks wasn't as effective either. It's about the trade off. I mean was Chris Byrd's best weight really 210 pounds? Was Jones any better at 199? Did Holyfield magically become better from 190 to 210? Clearly Rocky could add 20 pounds of muscle if he desired. The question is then just a matter of how competitive he'd be at the top level.
Putting on weight was never a problem for Marciano, after all he looked like a stuffed pig after he retired, I guess around 250 pounds. However he knew better than all the so called "experts" that his B E S T weight to suit his brawling style was between 180-190. Case closed.
I dont see anything unusual about these numbers to be honest. 185-190lbs was a fairly typical weight for a light heavyweight who was still active in the division, to come into a heavyweight or catch weight fight at, in the eras before bulking up became part of the programme. Also if Spinks was killing himself to make the light heavyweight limit, I rather doubt that he would have come into a title fight at 175lbs.
He ought to be able to maintain it over twelve rounds at that weight. I dont think that would be a problem.
This doesn't translate to other sports. Galen Rupp didn't break 28 minues for the 10K when he weighed 148. At 134, he broke 27 minutes. Seb Coe got down to 119 pounds to set his 1.41 record in the 800 meters. I could mention dozens of sprinters and distance men who maximized their strength to weight ratio for what was essentially higher output. You don't gain strength and output together, if you are already are a world class athlete. You simply can't have your cake and eat it, too. He may be stronger but he is not going to be able to produce the same output. Now, the topic of whether he would be able to display the same output against a world class superheavyweight jab is something entirely different.
For what it's worth, Eddie Futch (trainer of both fighters) said before the fight he thought Spinks made a mistake in putting so much weight on.