Not sure if you're still wondering this, but as far as I can tell, these "X counts Y% more" effects mean something like this: "For the purposes of bonuses that rely on the number of X, that number is treated as being Y% greater." This doesn't apply to their inherent revenue or other benefits, nor does the bonus count towards trophies.
Not sure how it interacts with artifact discovery chances, challenges, or other unlockables, but I suspect it doesn't apply to them either. Instead, spells like Night Time or Lightning Strike which determine their effects based on the quantities of certain buildings, or upgrades like Building Jungle (Druid 3,3) will benefit.
75% isn't a lot for mercenary builds, but if you're using multiple relevant components in your build already, it might be worthwhile.