As I recall from a couple of trips, mid or late August is a great time to go: warm days, cool nights, virtually no bugs, and the crowds are tapering off. Of course, where you're planning to go, crowds are never an issue anyhow.
I agree with what everyone else has said. I've never hiked the Greenstone down toward Windigo; I have done most of the trails on the "developed" end of the park. It seems strange, but the 6 or 8 miles per day recommendation is really pretty accurate. It's not hard walking, but it's very rocky and, for some reason, really eats up the time.
This is still about my favorite place to go.