As an Illinoisan, I might weigh in... From polls I've seen of Chicago NCAA fans and St. Louis NCAA fans, I'd say the breakdown is like this:
Chicago: 40% Illini, 20% Notre Dame, 30% Other Big Ten schools, 10% other
St. Louis: 65% Missouri, 25% Illinois, 10% other
Would UI get booed at Wrigley Field in Chicago? Absolutely not. They'd be cheered wildly ... but they might get some boos in St. Louis.