It's still early so the numbers aren't that compelling yet, but according to Sagarin we've played the 69th toughest schedule so far, while Wisconsin's SOS is 148.
Last time I checked, there were 120 D1-A teams, so, for what it's worth, there is an ENTIRE DIVISION of college football that has played a tougher schedule than them.
Actually, not really. Some IAA teams are ranked higher than IA teams.
So, this prompted me to sort Sagarin's schedule ranking just by IA teams, and you weren't far off with your statement -- Wisconsin has played the 113th weakest schedule out of 120 D1 teams. Purdue has played the weakest. FTR, Nebraska listed as 62nd of 120.