Actually, I'm looking at putting some P90's in another guitar I own, and I'd like to understand how the output of the two guitars will compare, when I switch guitars during a gig. The output of my Cutlass may help me choose my P90 p/u's.
There isn't really a standard for "output", since it depends on many factors including what you use to drive the magnetic field of the pickup and how you measure the resulting response. Some pickup vendors quote DC resistance, some quote mV output with a given string deflection, some talk about inductance, etc. Not usually apples to apples, even if you're buying pickups from the same vendor.
Anyhow ... MM doesn't sell the pickups separately, but from what I can see they're aiming for "vintage spec" pickups. Odds are, any P90 you choose is going to have more output and inductance (more mids) than a vintage-type single coil.
That's probably not much help. Maybe someone who has a Cutlass can weigh in.