Time travel doesn't work and suggesting it's easy is comical. No, it's hard.
For the same reason I wouldn't think about how Shane would have done at straight pool on the ten-footer on slow cloth, I am not very interested in how Greenleaf would have done in today's game. But given all the advantages available to players of the modern era and time to acclimate to the conditions, Greenleaf, a winner of 19 world championships when there was just one world title available each year (not three like today) would, in my opinion, have won easily over Shane, whose record in world championship play falls far short of what one would expect of an all-time great.
As I've said, the level has risen. The players of yesteryear, playing at the speed they played at back then, generally speaking, could not stay with the great sharpshooters of today. There might be fifty, even one hundred, today who shoot as straight as the superstars of 1940-1960. Yes, we are watching the best cueists that ever played pool right now, but greatness is measured in major titles, and when it comes to GOAT discussions, especially world championships.