A excerpt from Pollack’s In the ring with Corbett: “In his 1925 autobiography, Corbett recalled his observations of a 26 year old Sullivan. He said that a young Sullivan was much different from an old Sullivan. The aged Sullivan was thought of as sluggish, heavy of foot, standing in one place, and only having a right. However, Corbett said that prime Sullivan was a terrific hitter with both hands and was pretty quick on both his feet and hands, as fast as heavyweight champion Jack Dempsey of the 1920s. Still, Corbett also felt that Sullivan was not a highly skilled fighter, calling him a member of the “slugger class.” (Page 14) What’s even freakier is that the Sullivan that Corbett saw fight George Robinson in 1884 was estimated to weigh between 204-219 pounds, so he could be as much as 30 pounds heavier than Dempsey while still being as fast, at least by Corbett’s assessment who didn’t have much of a reason to boost a guy he regularly criticized as less advanced.
Joe Choynski who we have film of, said that Sullivan was faster than him, and hit much harder. Given the subsequent testimonials surrounding Choynski's power, that is quite a compliment.