More about the VP-1. I have three of them, each has its own 283. I also have a flash meter. If the VP-1's little knob is turned as far as it will go past -5 it will give a repeatable -6 stops.
I've changed my mind about this, mainly because of the difficulty of setting a VP-1 exactly at a marked output level (or my ineptitude), but from -1 down they give a GN that is the marked number of stops down from the flash's rated GN (120, ISO 100/feet). At full power all of my 283s are a bit more than 1/2 stop down from rated.