Even +% could be okay, look at EUIV. -25% coring cost, +33% national manpower, etc. - bonuses are significant. You can feel the impact. +5% worker output could be felt in retrospect maybe after game will end...and only if you would dig into that for some reason.
There shouldn't be a choice between +5% slave output and +5% amenities or -5% housing need but sbetween, for example, being able to buid Mega Forges, +1 to strategic resources output, unlocking powerful espionage actions, having intel(or something else) with empires you have trade agreement with. Unlocking some actions while locking others as a tradeoff. Something unique.
The difference is that it's hard to stack modifiers in EU4. There aren't very many things that give coring cost reduction (other than taking Administrative), and many things that give manpower (other than Quantity). All the things that tend to do that are either nation-specific, or religion-specific, and aren't available to everyone. If coring cost reduction was prevalent in a bunch of different idea groups, you'd have the same issues.
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