The broken JMT speed record is a remarkable achievement. Here is an interesting story by someone who made a valiant attempt to do the same thing a while back. It will give you a pretty good sense of what lies in store for you if you attempt to break the existing record.
Incidentally, when I hiked the JMT, I had to dig very deep just to throw down nine miles a day!! It's incredibly beautiful but quite rugged, snowbound and sometimes icy -- and as you well know, resupplying is rather challenging too.