Surprise, surprise. The QI euphoria counter is apparently bugged. It says I only have 600 provided euphoria, but I have a triumphal Arch that provides 675 by itself. Also have a decoration for 75 more but I don't get that either. Looks like all I will get is the attack boost. My population is 350, so with 750 euphoria I should be producing more than 120%, I think. What a piece of work this feature is! Still hate it, in case Inno is interested in feedback. It's hard to do literally EVERYTHING wrong, but bless their hearts Inno somehow finds a way.
EDIT: Someone explained to me that decorations in QI take away euphoria instead of adding it, even though, unlike every other negative happiness building in the history of the game, they use the happy face with a positive number in white and list it under "cost" instead of using the mad face and a negative number in red. Just another juvenile "gotcha" from the developers.