In this scenario Sullivan turns pro and has the same career length as Dempsey, and faces the same competition he did. However, he also reaches his peak at the same time as he did in real life, and his peak lasts the same time amount of time. How does this change things in the heavyweight division from 1915-1927?
Beat all the best white fighters, find some exuse not to fight the best black figjhers. Amd still loses to Tunney in 1926.
He will get all Dempsey's work win wise done by 1923 before he takes his 3 year break like the Real Dempsey did. I think people sometimes forget Sullivan himself took years off his rein as well.
I think, in their respective mileu's, Sullivan was [by fairly far] much more averse to 'mixed-race' bouts than was Dempsey. I think there came a point where Dempsey and Kearns would have been delighted if a promoter (especially Rickard), a State Athletic Commision and a State Government (legislative and executive) could have gone all in for a Dempsey-Wills mega-fight. I think Sullivan, in Dempsey's day, might have hit big time a earlier, would have been about as dominant, may or may not have gotten 'Meehan'd' (with no one giving any more a particular turd), would have held it together until Willard met him; if he then took a 'I'll train if someone makes me think I need to' attitude, Dempsey's Miske-Firpo opponents might -- possibly -- have attitude adjusted him for the better. JLS had more stage presence than Dempsey, so the theatrical/circus/touring life might have been more of a distraction. If Sullivan is off three years after Firpo, I think he'd be a f@#*ing wreck by the time he met Tunney; more so than Dempsey, as much as Jack went back.
But, really, it all depends on when 'the diamond gets polished'. I'm sure if Dempsey was getting the quality of training and support, at eighteen, which he actually started to receive at twenty one, he also could have 'hit the big time' earlier.
It took Sullivan about a year to become a relevant contender, so if he went pro when Dempsey did he would be in title contention by at least 1915