Stunning for an average of 10 rounds doesn't seem that inferior to me...
Except it's only usable 1/day, as opposed to 1d4/3d4 Con every 1d4+1 rounds!
Anyhow, I'd roughed in Challenge Rating 14, so I'm OK with it if you are.
That would make them SR 23, going by the CR+9 you wanted.
Hmm, a nice round SR 24 or 25 would be more aesthetically satisfying for me, any preferences or objections?
Let's go with 24.