I have discussed this with Charlie Ursitti, who was Mosconi's manager. Statistically, Charlie offered, Mosconi produced more one inning games than Greenleaf, but he also offered that Greenleaf had more games of two innings or less than Willie. I've met more than a few over the years who watched both play, and it's nearly too close to call, but the feeling is that Mosconi had a very small edge. Noone who thinks Greenleaf was the greatest ever is out of line, however.
Players of Greenleaf's era are largely forgotten. Believe it or not, there was a woman straight pooler whose speed was about the same as that of Jean Balukas. Her name was Ruth McGinnis and Irvng Crane told me that she beat Greenleaf on more than one occasion in exhibition matches and that the two of them toured together. If my memory serves, her high run on a 5 x 10 was in the 130's, a pretty tidy accomplishment that few women who've ever played pool could hope to approach.
Greenleaf or Mosconi? McGinnis or Balukas? These are very hard comparisons for anyone to make.
... but it's still fun to try.
PS Ruth's 130 on a 5 x 10 has no confirmation, but she did run 125 in competition on a 9 footer and an 85 on a ten footer according to her writeup on the BCA HOF page.