He may have gotten a fragment of the title after Spinks leaves,but don't think he would do what he did.
Not sure what you mean. Are you saying his impact at 175 wouldn't have been as significant as it was at Cruiser?
Not much. Spinks beat him to unify the division. Dwight pulled out of a rematch in '84. With Spinks being unified champion, there was nothing there for Dwight until Spinks vacated in mid to late '85. By that time, Dwight was winning the WBA Junior Heavyweight (Cruiserweight) title. Then, had the epic fight with Holyfield. If he had stayed at 175 lbs until Spinks left, he could possibly could have had a good fight with Marvin Johnson. And he probably could have won one of the vacant titles. But, I doubt he'd dominate the division at that point.