I looked up the BRX250 on Elinchrom web site, but unfortunately they do not list any expected f/stop at any power levels. But we can start with BRX500 has minimum 32 w-s setting, while BRX250 has minimum 16w-s setting.
So I went to Paul Buff web site to get comparable outputs. Buff says at 5' from subject
- 40 w-s should result in f/2.8 + 0.9EV, and
- 20 w-s results in f/2 + 0.9EV
So we can conclude that minimum power on BRX500 results in about f/2.8 + 0.5EV...do you really think you will be needing less power than that?! You can always put a -1EV or -2EV ND filter on the camera lens, rather than the more cumbersome technique of gelling the light sources.
And if you use softboxes rather than the standard reflector, you will reduce light output by at least -1EV (and possibly more).