I repeated the experiment I did before - but this time with a softbox as kicker / rim light.
Setup: see diagram. Main light always at f/4.0, kicker in 1) at f/2.8, in 2) at f/2.0 and so on.
I repeated the exact same setup with a snooted SB-26 as kicker and found out that my softbox eats 2 stops of light. Not too bad I think. I'll post the results - and especially conparisons between snoot and softbox kicker - as soon as I have the time for PP. Spoiler: The softened kicker is comparable to a bare strobe that is one stop darker.