I think Mosconi may be in the top 10 best-ever, but absolutely not THE best pool player ever. IMO he didn't have the all-around game repertoire, didn't really play for the cash to speak of (which is the true determining factor in who's best), and played in an era when a lot of the best players (Don Willis for example) didn't enter tournements because the $ was elsewhere. I would, without hesitation, put the following players ahead of Mosconi: Reyes, Parica,Luther Lassiter, Harold Worst,,Ralph Greenleaf (who Minnesota Fats said was the best 14.1 player he ever saw),Mike Sigel,Buddy Hall, Alan Hopkins, Eddie Taylor, and a bunch of players (Strickland, Archer, Jim Rempe,Ed Kelly, Ronnie Allen, Jersey Red, Jimmy Moore, Don Willis, "Rags" Fitzpatrick et al) as his equal, at least. His record of running balls in 14.1 is very much in dispute (reliable sources have repeatedly stated Babe Cranfield ran over 600), was made on a 4X8 table (how many would Willie run on a tough Diamond 4 1/2 X 9), and IMO has remained as "the record" because of his exalted status with Brunswick (who OWNED pool for years) and the billiards hierarchy. I put Mosconi in the same category as Mickey Mantle: when I grew up, Mickey was all we heard of, he was "the man", the best ever, etc. We now know that, while he was a terrific player, he was not the equal of Willie Mays. Sometimes the myth exceeds the reality. While there is no question Mosconi was a stellar player, to say he is the best ever because he ran a lot of balls and won a lot of tournements, diminishes the achievements of the true "best ever" contenders (IMO Reyes and Lassiter). To the original question, I think if Willie could play using today's equipment, he would run less balls but be one of the top 14.1 players (but he wouldn't like facing Hohmann, VanDenBerg, Schmidt and some of the new young guns).
Sorry if I came down a little hard on Willie...hard to make a player debate without doing so.