"Has anyone hit the JMT from Sphinx creek?"
That sounds painful and convoluted. You're going to climb over Avalanche Pass, then Colby Pass, then down and up and around.
There must be a dozen ways to get to the JMT. Kearsarge Pass? Baxter? Taboose?
Up Copper Creek, over the Monarch Divide, down to the Middle Fork of the Kings, then up to the JMT.