You may want to ask this question over in the Universal Forum as well.
In general, Reports over the past year have said that Saturday tends to be the busiest day at Islands of Adventure (Vacationers and locals), while Sunday tends to be one of the slower days. Since this will be the 2nd fall season since Harry Potter has opened, I'm not sure if anybody would have a good historical idea of the crowd levels though. Prior to HP opening, Universal could be very easy to deal with crowd wise during the "off season", and last year reports were the place was pretty busy even during traditional slow periods because of the crowds to see HP after it opened.
Once we enter the summer season, we may have a better idea of how crowds are comparing to last year's, and therefore get a better idea if things will be better than last year crowd wise, or if Universal is doing a good job at maintaining their crowd levels from last year.