I wouldn't have said that Dempsey would have been prime even two years earlier.
Dempsey's prime was 1919-1922 for me. We complain about the state of the heavyweight division today, but frankly it was shocking that Dempsey held the title from 1923 to 1926 without one defense! It could be argued that Dempsey was showing signs of slipping in his thriller with Firpo in 1923.
Its no surprise that Dempsey lost to Tunney; throughout that time Gene had been very active, sharpening his skills against the likes of Greb (several times!), Carpentier, Delaney and Gibbons.
History forever shows us what a two or three year layoff can do to a fighter. When Dempsey met Tunney he was rusty and had lost the pace and sharpness of his prime years. Even then he did scare Tunney a fair bit, certainly in the second fight!
As for who would have won if they had met prime for prime, I'd always maintain that Tunney would be a tough fight. The man had simply superb boxing skills, a terrific chin, an iron will and unmatched ring intelligence. That said, I would shade prime Dempsey over prime Tunney under 1919 rules. 'That' Dempsey had a ridiculous level of pace and hand/footspeed that would seriously scare Tunney......and if Jack did manage to knock Gene down (hardly a rash assertion considering the 1927 rematch!) than Dempsey would simply stand over Gene and bludgeon him the moment he tried to repeatedly stand up!!!