I have no idea how long the releases will last. I have boated it at 300 cfs and I honestly can't tell that much from 1500 cfs. It just takes longer to float.
The 13 miles last will take about 4-5 hours with lunch break and a few beers. I'm sure with some paddleing you can beat that time.
There is no real whitewater in the canyon, just a pretty place to be in your boat. I suggest a nice lunch with beers and enjoy the scenery. Take your playboat. The play is minimal but it makes the hike down easier.
I hope this helps