Excuse me while I laugh my butt off!!! I'm sure someone, somewhere, timed Mike Piazza at a 1.78 but I would have to seriously question the accuracy of the watch or method of the timing considering Piazza has an average arm at best and below average footwork. A 1.78 pop time is in the Johnny Bench, Pudge Rodriguez category. I guess someone clocked Jamie Moyer at 97 on the gun too!!!
Speaking of pop times, there is a HUGE DIFFERENCE between times registered in a game and times registered in a workout setting. In tryout camps or workouts, catchers tend to cheat more and they KNOW they are about to make a throw to 2nd base for the watch. As a rule, I add at least a tenth of a second to the time, if not more, depending on the circumstances. For example, if a kid throws a 1.9 in a tryout camp, I figure he's closer to 2.00 to 2.05 in a game.
While I obviously haven't seen every throw by every catcher that has ever played in the big leagues, I have had the opportunity to get pop times on a lot of good ML catchers over the years, either as an amateur or in professional baseball, and the best I've had on my watch was Pudge at 1.83. The only other guys I've timed in game conditions under 1.9 were Charles Johnson, Ron Karkovice, and a journeyman named Alberto Castillo. I would say on average, most of the guys I've clocked were in the 1.95 to 2.05 range and I've even had a few that have thrown over 2.1.
Now in tryout camps, I'll get 1.8s (even high 1.7s) every once in a while and times in the 1.9 range on a consistent basis. But when you take into account the anticipation of the catcher, no hitter in the box, and the fact he has no consequences for throwing the ball away, you have to assume he won't be able to duplicate the same times in a real live game situation.
As for what is a good pop time for a particular age? That really depends on the projectability of the player in question. I could easily say that 2.1 is good for a 15-yrs old but if there isn't much projection in the body or throwing arm, he may never improve. Meanwhile, another 15-yrs old, may have a live body and an electric arm but lack the footwork and strength to throw better than 2.2 or 2.3, but has better future projection.
I'll leave it like this, if you have a young man who can throw under 2.1--with accuracy, can block and receive, knows how to run a game & handle a staff, and can work a count and make consistent contact at the plate, he'll probably play for a LOOOONG TIME!!!