I agree that it would be nice to know if it covers the entire domain, but depending on how you mean "noticeable" then I don't think I agree there.
Aside from telling the contest winner what their monument is and what the benefit is, I think it would be more interesting if mechanically you couldn't actually notice a difference in game. It would turn it into kinda a local legend that has the potential to slowly fade into obscurity, or when people come visit the town of bustling trade and the locals all say it's thanks to the balance monument that gives everyone luck in their deals the skeptics that weren't here for this contest can scoff in ignorance at the superstition.
I guess to clarify, I don't want to be able to examine the monument and have a ui tell me it adds a bonus to the settlement, if that's what you meant by noticeable.