The only thing you can say for certain is that the fight that actually took place means next to nothing. Even so, my money would still be on Johnson to outbox, out-grapple, and generally frustrate the hell out of Jeffries, unless Jeff really lands clean and takes out Johnson as he did Fitz and Corbett.
Johnson still wins .Jeffries at his very best never beats Jack at his peak .Johnson a few steps above.
Johnson was appreciably bigger and stronger than both Corbett and Fitz,if he landed clean that doesn't necessarily mean Johnson goes anywhere,imo.
When was Johnson's prime? He was shown up by lesser men than Jeffries fought as champion from 1909-1914. He lost some while Jeffries was champion 1899-1905 who Jeffries bettered by a wider margin.
Always hard to really say, It depends usually on how bad of condition the older man was. In the case of Jeffires he was inactive for 3 years overweight by about 80lbs and at 36 not too clever to come back without a tune-up and a chance to gain strength after losing a significant amount of weight. I would have to extrapulate that Jeffries would have been a much more lively foe but Johnson was also pretty seasoned,nimble,advanced style for the time & strong. Jeffries next to Ali vs Holmes was probaly the worst conditioned of the old Champs who lost to upcomers or New Champs. Joe Louis vs Marciano had only lost one fight to ATG Charles and was on a win streak & although not the Brown Bomber of the past,still formidable, Dempsey had been inactive vs Tunney & odd circumstances surrounding the Long count but still dangerous as well. Ali vs Holmes was a semblence of Ali who was completely shot. Tyson vs Lewis not a prime Mike but still dangerous. Some of these fights were style advantages and some were because the former Champ was a shadow of the past. Jeffries was not near his prime but how would a prime Johnson handle a prime Jeffries? Nat Fleicher told me Jeffries was a beast and Johnson was a master boxer and for that time Im sure they were. This one was a mismatch prime vs prime would be a different fight and more than likely very competitive.
It is impossible to say really, because Jeffries is such a loosely defined fighter, and even with Johnson we are missing significant parts of the puzzle. A lot could depend upon the rule set, with a longer fight potentially favoring Jeffries.
Jeffries went 20rds Choynski 20 rds Ruhlin 20rds Sharkey 23 rds Corbett 25 rds Sharkey Johnson went 20rds McVey 20rds McVey 20 rds Mcvey 20rds Gardner 20rds Klondike 20rds Denver Ed Martin 20rds Hart 20rds Moran 20rds Ferguson 20rds Everett 20rds Griffin 20rds Griffin 26rds Willard I don't see any basis to assume Jeffries would be favoured by a longer distance.
More rounds, is more time to catch Johnson, or break him down incrementally. If Johnson only has to outbox Jeffries for ten rounds, then his task is fairly straightforward on paper. If he has to do it for twenty rounds, then the initiative is more likely to shift to Jeffries.