The weather in the Canadian Rockies in September isn't always as nice as in July/August but if you are prepared to "hole-up" for a few days if you are hit with an early winter storm and show some discretion when conditions are poor, then you will be OK. I think you can count on a least one multi-day snow storm and many mornings of -0C weather in September, especially if you are on the Icefields Parkway. The snow may be significant on the passes, but I've found that even just frost and frozen sleet on the highway shoulder in the early mornings can be pretty dangerous, even terrifying on descents. Don't even think for a minute that you're going to be loaded touring if there is snow on the road.