As far as against Frazier or Foreman I think he would surprisingly well. Frazier never really impressed me outside of the Ali fights. Foreman is overrated as a "bulldozer". The way he is described on here you would think he just bum rushes in and starts clobbering his opponents. He hit hard and was a pretty good fighter but his stamina was **** which he would pay for against Tunney. Don't think anyone here is going to argue that Tunney had a much higher ring IQ and better mobility than Jimmy Young.
https://www.reddit.com/r/Boxing/comments/1ya9vd/my_fights_with_jack_dempsey_by_gene_tunney/ This shows how dedicated Genne Tunney was .
Dempsey had numerous exhibitions post-Firpo fight over 3 years getting "ready" for Tunney (1), Jack's next real fight. Both Tunney fights were scheduled for 10 rounds, indicating Jack knew his own legs were only good for a short fight.
I think he's a bit overrated at Heavyweight and if anything underrated at 175. Certainly a great great fighter. I don't really believe he would beat the likes of Louis, Marciano, Foreman and Frazier at their peak however. Dempsey was well past his best when Tunney rolled him, for me it doesn't give him as much (heavyweight) leverage as some. Spinks beat Holmes at a similar stage with canny boxing and i'm certainly not about to pick him over Foreman, Frazier et al. Charles beat Louis late career too. Three great fighters but against peak heavyweight greats it's a gonna be tough.