Dempsey would need to catch Joe off guard. He wasn't the same level of fighter, realistically. Power for power, sure, but Joe could get Dempsey equivalent power without windmilling punches. I don't think comparing Firpo's power, which had little technique to brag of to Louis is applicable. A tight 1-2 from Louis may be the most explosive single combination ever from a boxer. And Dempsey would eat a lot of them trying to get to Louis and probably be knocked out relatively early. To answer the OP question: Dempsey KOing Louis is possible, I'd rate it about 5% or less.
Even his short shots were big whole body movements to me, he didn't look to have clean technique. He would routinely square up and drop both hands to full-on swing. But hey, I potentially dksab. Just my take. I doubt they would get home on Louis, even in the clinch.
Im not sure why people keep saying that Conn weighed only 169 against Louis or that even Carpentier weighed more than Conn against Dempsey. Neither is true. Yes, anecdotally people have said Conn was lighter than his official weigh in weight but thats simply not true. The weigh in was filmed and was conducted in front of dozens of reporters. Nobody disputed either fighters weights despite half a dozen men from both camps staring over the weights and Louis weighing in bare ass naked. In fact, both men stepped on the scales numerous times for the motion pictures which still exist today and easily refute the idea that the weights were falsified. Keep in mind that the same thing was said about the Dempsey Carpentier fight, that Dempsey's weight was lowered and Carpentier's weight was increased to make the fight look more competitive. Yet, Dempsey weighed basically the same as he had for his last four fights and Carpentier was right around where he weighed for his last three. When Conn weighed in at 174 for Louis (two pounds more than Carpentier for Dempsey) he was at his lowest weight in his last five fights. While its true Louis was at his lowest weight in his last 6 fights over the previous year, his average weight for those six fights was 202.5. He weighed 199.5 for Conn. Not unusual when training to be lighter and faster when going up against a lighter faster fighter. And again, this is a guy who weighed in naked. People pointing out that Louis was rocked by Conn shouldnt be so quick to throw stones. Dempsey was made to do a drunk dance every bit as nasty as Louis' when Carpentier tagged him in the second round of their fight.
Dempsey çould hit, I have no doubt he could knock Louis down or hurt him at some point. I do believe one of the things about Louis that gets overlooked is his recovery abilities. Joe was able to survive after being hurt and comeback to knock out his opponent several times. I guess it depends on just how quick Jack was in his prime.