Just got off the upper this weekend, it was low, but certainly do-able...for the full trip, 600 is low, but also do-able...the last 11 miles are just a drag.
September is a difficult time of year to predict and guage. If you were to look back to the last 100 years or so, you would see that often September has some of the most violent spikes of any month. As recent as 2003 or possibly 2001 the river shot way up into the teens during September.
There is a gentleman on here who is in charge of the flow one way or the other, or certainly has the inside scoop...I will let him field this question, as I am sure he will as soon as he sees the post. He always offers insight.
My suggestion would be to go regardless of the flow. I have been down in the lower canyon as low as 350 in duckies for a 4 day lower trip...we had to work hard to cover 20+ miles nearly everyday, but it is always worth it. September is an awesome time to be down there.