Passed balls should be a rarity - the equivalent of an error by an infielder. More important would be the ability to corral a wild pitch (those pitches which are not controllable with ordinary effort). Once you get passed both of these (treat them as prerequisites) the catcher needs to be able to control the base paths and therefore must be able to throw out runners. I'm sure if that opposing catcher had a 1.7 pop time but had several passed balls, a coach may be able to coach him up on receiving and treat pop time sort of like foot speed - either have it or don't. If your catcher has a very slow pop time (hopefully that is not the case) then perhaps he still had folks steal on him with his throws simply being too late for the tag.