I decided to go back through the last few years and figure out how handsomely free agent catchers have been rewarded for their work. What follows is a plot of their most recent VORP leading up to the contract, along with the annual value of the contract itself. It's an inexact science based on a reasonably small sample, but it should give us a quick ballpark estimate of Johjima's target performance.
(Note again: defense not included.)
To "earn" his base salary of $5.5m per season, Johjima would need to put up a VORP of 29.7. Ivan Rodriguez stood at 24.4 with a .276.290/.444 batting line, so given enough playing time, Johjima shouldn't have too much difficulty exceeding that figure.
$16.5m/3yr isn't the absolute steal that $8m/2yr looked like, but the guy's downside would still be a slightly above-average performance, and his upside is huge. And let's be honest, who really cares about the money, anyway? If Johjima's contract actually counts against the payroll, I'll be shocked (sometimes the fuzzy math comes in handy after all).