You have to understand that Spinks' chances are much better with a prime Foreman because a late-eighties Foreman was fatter. Fatter means bigger, bigger means more mass, mass equals more force, more force means more speed. Therefore, because he is bigger, he is stronger, because he is bigger and stronger he is faster, so Spinks would be fighting a faster and more powerful fighter in the late-eighties.
I see this like a repeat of Fooreman-Moorer; but with a faster, smarter Moorer who doesn't let himself get caught. A lumbering George relentessly pursuits Spinks round after round looking for a puncher's chance. But Spinks is too fast and nimble and plays it safe staying out of range, sticking to a very cautious fight plan while snipping George for cheap points, eventually coasting to an UD.