You're right Lozz, the JMJs are supplied new with fender long scale stainless flats, however I'm led to believe that the cobalts are more fragile than steels and might not survive the winding going around the string post. Maybe if any of the sets I have on a thunderbird gets a bit dead sounding, I'll experiment with that set — I wouldn't want to risk it with a new set though.
I've just remembered I have a set of 30" Cobalt flats which should fit my Sienna sunburst PJ mustang as the body isn't string-thru, so I can see how they sound on that bass.