I appreciate a lot a thread like this by a new member. It could seem a troll instead Is very genuine. I remember I began the adventure in this forum with something like that. Welcome.
Gaining weight didn't really affect him, none at all tbh. Mackie Shilstone, who worked with spinks for years, changed his diet and exercise program, when he moved to the heavyweight, to gradually build him up to his natural weight without losing speed and agility. And it didn't end in failure. Source: https://www.latimes.com/archives/la-xpm-1985-09-29-sp-18830-story.html He also said "He [Spinks] ran the 880 in 2 minutes 45 seconds, not bad for a 200-pound man, Michael put on weight, but he has only 7.2 percent body fat. And the power lifting we did was designed for his stance in the ring. He can still touch his knees with his chin.'' Source: https://www.nytimes.com/1985/09/21/sports/spinks-at-200-for-title-fight.html He also helped spinks gain weight even more before the tyson fight and again without losing his speed. The claims that Spinks was past it or his legs were shots are easily the most difficult information to find, my man. But if you can source it, i will STHU.
No I don't, and I don't think that there is a good argument for it either. His early accomplishments were tremendous, and surpassed any other young heavyweight, but a number of heavyweights had clearly surpassed him when he retired. Once you throw it open to other weight classes, then he is up against Langford, Greb, Armstrong, Robinson etc. Good luck navigating that lot! So the answer id no, but he still accomplished a hell of a lot!