We lived in EP for 31 years and I worked in accommodations. So, I know a "bit" about the fall elk rut and crowds. The crowds are insane in Rocky during late September when people go to EP to look at the fall colors and watch the elk. On the weekends the elk jams are as bad as big city traffic.
The crowds thin some after the aspen loose their leaves which they have for the most part. As long as the weather holds, weekends are still busy but not as insane. When you are planning to be there weekday crowds and traffic are not bad at all. That time of year weekday visitors are the newly wed and nearly dead.
The elk will still be in full "song." This will sound nuts, but the best place to watch elk is on the nine hole golf course right in town. Moraine Park is a great place to camp and watch wildlife.