One of the greatest light-heavies,but who literally got beaten down any time he moved up to heavyweight. He'd only carry a few extra pounds when he did this,but just suppose that he'd had modern weight increasing training methods that Michael Spinks had. How would a 200-210 lb Foster have done ? Imo it would have made some difference but not much. He would have beaten Doug Jones and possibly Ernie Terrel,but not Ali and Frazier. Even though he'd give them more competitive fights than he did in reality.
I have somtimes wondered if he could have done better at heavy in his own era with more astute matchmaking. The extra mass would have helped him to take a punch a bit better but would not have compensated for his stylistic shortcomings at that weight.
Stevie, you have got the right idea in assuming that Foster today would be able to weigh in at a functionally heavier bodyweight. But wouldn't the heavies of his era also be heavier, given the same advances? So, we are back to square one. If you meant a Foster given modern advances, while his HW opponents of his era remains the same, then I guess we might see some changes in his performances. Would he be able to beat Doug Jones? Possible. Terrell would still be my pick against him. One very good guy he would likely beat is Zora Folley. Foster gave him a competitive fight back then. Frazier and Ali would be beyond him. Though a 200-210 lbs Foster might be able to hurt Frazier in the first couple of rounds. The point is, a 200-210 lbs Foster today would be fighting guys like the Klitschko brothers who would be 240-260 lbs.
Foster also lost to small heavyweights,Doug Jones for one. Bob despite his height and power was a lightheavy, he lost his advantage when he moved up, I think Trinadad, Delahoya and Michael Spinks, great fighters suffered from this also. Some guys just dont have the frame to move up....Evander Holyfield did have the frame...so did 5"9 James Toney ( who would figure) Duran was a natural Lightweight but his greatness and skill level made him competitive beyound 135
He outboxed arguably the greatest heavyweight of all time for the better part of 8 rounds,before going down.