I assumed everything else was equal when I made the .300 vs .295 hitter but failed to say it.
If you look at Sale's sample size, the 1200+ PAs is like two full seasons for a hitter. He's about 3/4 of a run better with Leon than all the other catchers combined. Of course, there could be other factors that helped create that gap beyond anything Leon did differently, but when you don't see any other starting pitchers with much worse numbers with Leon, I think one can accurately surmise that Leon makes a big difference with some pitchers and maybe not much with others, but is rarely negative with any large sample size pitcher. Yes, the Vazquez/Swihart sample size is not that large, but everyone else combined is!
Maybe it's not 0.75 just because of Leon, but even if he was responsible for 0.25 to 0.50, that is far and away enough to offset his .450 OPS on offense.
The gap with Price, to me, seems even more striking when you figure his years with Boston have been spotty, and he was not in prime years, yet he still blows away the rest of the catchers combined than when he's with leon.