I think the relevance is to thermal fusion, since the steam cycle is so lossy. The heat surplus has to be enough to push through enough to generate the input to the reactor from a steam turbine. Then to get any useful power requires more. So I’ve seen suggestions that Q needs to be around 5 for a steady-state thermal fusion system to be viable.
For FF, all the excess from the direct electric output is usable/salable. So Q loses its relevance, IMO.
