It all depends on how well you can run technical water and if you don't do it well, much abuse you're willing to let your boat take.
The outfitters were running 12' and 14' boats down there in years when the flow never got over 500 cfs - and was usually around 300.
One of my moments of inspiration was the trip I started off sharing the launch at Johnson Village with a guide as she loaded up a 16' gear pig. She packed a huge mound of gear on the boat to support a family on a 4- or 5-day trip all the way to Canon City. We all launched and leapfrogged for the first couple of days, taking out at Stone Bridge as they kept going. I always figured if she was able to run that huge boat with a half ton+ of gear, I could run it with my 14' boat carrying a moderate load down to 600 cfs.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse