I get f/5.6-8 with Vivitar 283 iso 400 no problem with 8 ft ceilings if the distance to actual subject isn't too far.
There is some falloff if you go into a corner.
I usually go straight up where the lightstand is out of the shot and that gets you plenty of even light even with the one vivi 283.
The problem I came across is it looks great at first compared to direct flash but after awhile the look gets boring.
It just is too even for any good light modeling on faces but works for everyday non portrait type shots.
When I go directly up I use a small card on the flash to throw a bit of light into eye sockets and for fill.
400w/s should be fine for even medium format if you don't have cathedral ceilings to deal with.