I think scoring in these events is dependent on both weather and whether the club actually takes the time and wants to prepare the course. For the Fighting Illini Invitational at Olympia Fields, we don't do much because we get a lot of guest play in September and growing the rough out would make for 6 hour rounds. However, the weather was very dry in the weeks before, and the course got very firm and fast. Balls were bouncing into fairway bunkers, the players had trouble getting close to pins, and so, even though we had 7 of the top 10 (and 15 of the top 30 players) teams on golfweek's rating, +30 was the winning score, and the course averaged more than 75. If there had been much wind, then who knows (aside from the course firmness, scoring conditions were very good).
The year before, we got flooded and the place was mush, and the winning score was +12, and the course averaged around 74, with a somewhat weaker, though still very strong, field. I suspect that if we wanted to prepare the course, growing the rough and keeping carts off it for a couple days before, it could play significantly more difficult in dry conditions. Same goes for Kingsley, Skokie and other venues.
Incidentally, Don Cross, the DOG at Skokie, has a terrific blog at
http://skokieturf.blogspot.com/ Among the terrific general discussion, he discusses preparations for the 2010 Western Amateur at Skokie, and they are taking it seriously. Fertilizing and seeding the rough, zooming up the greens all year, and even talking about rolling the fairways.