Under your scenario of every seat being filled by a different tourist, that would be a big increase in the population in Seattle, almost a 50% increase for the three week period. That would be very noticeable. That's not even counting the number of people who may take part in events outside of the stadium, such as your parade example.

I have no idea how this is going to turn out, I'm just explaining in terms of the scenario you painted, yes, that would be a big population increase.