There's possibly an upper threshold but I agree, I don't think they're near it yet.
Many people come to IOA/USF specifically for Potter (I know some), some even call it the "Harry Potter theme park" and assume it's an entire theme park. Realistically, HP could have been an entire theme park, and if that's true, with Diagon+Hogsmeade, they're only about a third of the way there. (Financially, it makes sense to spread the HP IP throughout their parks to force multiple ticket purchases.)
We're considering Nintendo in all three parks (Mario-UFW, Pokemon-USF, Zelda-IOA)... To think the IP which changed the theme park industry and is Universals biggest cash cow deserves less than Nintendo, I believe, is very silly.