We will be there Sept. 3-10 as well. On touringplans.com (website of the unofficial guide to DisneyWorld), it lists the crowd level at a 4 that entire week (except Saturday the 3rd which is a level 7). Crowd levels usually range from 4 to 10, so this looks like a GREAT week to go. They have done a lot of research on this!
Quote from website:
How We Determine Crowd Levels and Best Days
A number of factors contribute to the models we use to predict both crowd levels and the best days to visit each theme park. Data we use to predict crowd levels include:
Historical park hours from the same time period in previous years
Disney's special events calendar (e.g., Grad Nights, Mickey's Not So Scary Halloween Party, etc.)
Legal holidays in the United States
Public school schedules (including spring break schedules for the 50 largest public school districts east of the Mississippi, plus Massachusetts and Connecticut)
Weekly historical occupancy rates for Orlando-area hotels
Central Florida tourism demographics
Actual wait time statistics gathered in the parks
Our own surveys of Disney guest touring habits
Disney's Extra Magic Hours schedule
Special events calendars
U.S. legal holidays
Outside sources (e.g., the U.S. Department of Transportation, Consumer Reports, etc.)
This translates into the following peak wait time at MK headliner attractions (Space Mountain, Splash Mountain, etc):
Level 4 35-45 minutes
Level 7 65-74 minutes
Remember these are PEAK waiting times for HEADLINER attractions. You can always get a Fastpass.