You could hike 20+ miles per day on the JMT, but why would you want to? As a former PCT hiker, I felt somewhat guilty about the pace we had to keep. It was like running through a museum or chugging a bottle of 50 year old scotch. If it wasn't for the necessity of making time on a long trip like that, I would have loved to average, oh, maybe 10 to 12 miles a day in the Sierra, leaving plenty of time for side trips or just lazy days by a stream. I ended up probably averaging around 18, with a number of 20+ mile days thrown in there for good measure, but it wasn't as fun as spending a little time at a lake, throwing off the pack, and exploring here or there.
But everyone has different goals, and I wish you the best of luck. I suspect you could do it in 10 days, but I think it might be more enjoyable if you could take a little more time and allow yourself to stop early if the mood strikes you. Granted, in August there isn't the snow to contend with, but still, it's a lot of up and down.