This would make it totally OP.... But Tholos was already such a crazy building that gave all the good stuff (Happiness, Goods and FP)...it whould be better for happiness rather than population
I guess that this is just Innos way of regulating how many of these buildings the players will be able to place into their city... Still I have seen cities with ten CN or more and no problems with happiness at all... If you lack the happiness, just build some regular cultural buildings - this just devalues the event building a little as you need extra-space.