I did Deerlodge to Dirty Devil in May/June 2006 in 15 days. Calculated it at 435 miles. We spent 2 nights (not in a row) on the boats floating- 100 miles in a sitting. The first section we did like that was from Split Mt. boat ramp to Sand Wash. Second time we did it was in Labyrinth. Everything between Green River and the confluence is completely flat, as it the Uintah Basin. Stillwater Canyon is beautiful. 15 days might be tough at less than peak flows. We spent 5 days between Deerlodge and Split Mt., 5 days from there to Green River, and 5 days from Green River to Dirty Devil, and we saw around 40k cfs in Cataract, so you know it was moving. Let me know if there are other questions I can answer for you. Have fun!
EDIT: I can't believe my wife had to remind me of the dam between the Deso/Gray takeout and Green River. It's nasty. We ran it just right of center, aiming for a bend in the drop, slightly missed my line, and made it ok. The kayaks sneaked far right, dropping right at the wall for the diversion channel. No telling what it looks like at flows less than 20K+. Scout. Be careful.