Mike Page posted something a few years after John made the switch. John dipped down for a while after the switch and then returned to the same rating.
I can also add that I had the great pleasure of watching John while he made the change. John plays out of the same pool room I do in Toronto. When the pain got so great that he had to take a hiatus from playing pool, he started playing a bit with his left hand.
John’s idol in cue sports is the great Ronnie O’Sullivan and he had previously worked on trying to play left handed well enough to not need a bridge on many shots. So he wasn’t starting playing left-handed from nothing, but my guess is that when he first started experimenting left-handed full time he was probably around 700 Fargo with his left hand. All of the knowledge was still there, of course, and 90% of the shots were fine. But when it came to a tester or a shot that required power, he did not have the same degree of confidence.
What I got to see were the countless hours he put in practising left-handed, building up the same level of expertise and confidence to be able to play tough shots under pressure, where the real champions need it.