Worth noting their first fight was before Sharkey peaked (before the first Jeffries fight), and I suspect the second Jeffries and subsequent fight with Ruhlun had taken a lot out of Sharkey. I still agree he was inferior to Sullivan though, though it's hard to say how analogous they are. Sharkey's better footspeed seems to have been important to his success. Hard to say how it compares to Sullivan's. I'll need to read more about Sharkey to figure him out more, a very odd fighter.
I don't think that Tom Sharkey was particularly similar to Johns L Sullivan. I get the idea that would be like comparing Bert Cooper, to Mike Tyson!
Jeffries I believe was just as big, strong and tough, as fast and quick as Sullivan, but Jim was more of an all around athlete so most likely moved better and could have avoided more punches than Sullivan would have, I just don't see John L. going past the 7th round, he may give him a great fight for 6 or so, but then it ends or starts going down hill very quickly.