In my experience, strobe photography is never about absolute power, but relative power against whatever your conditions present. As mentioned earlier in the thread, having a higher native sync (And not silly HSS) goes a long way in making small strobes put out big light on location. For example, I have noticed that several people complain on the forums that the Elinchrom Quadras are "Not powerful enough to overpower the sun". In my experience, it really depends on how you use them. With Pocketwizard Plus IIIs and the Nikon D800, I have managed to sync the quadras up to 1/320s, which coupled with an efficient light modifier such as the Maxilite reflector is enough to overpower the sun even here in tropical Singapore.
Here's an example at 1/250s and the maxilite. 1/320s would help cut another third of a stop of ambient.
Shot at 1PM.
Since you need to shoot 8 people or so, I would recommend the use of a highly efficient modifier such as the Paul C Buff PLMs (You'll need sandbags and strong assistants if you want to use them outdoors, though). The Einsteins and the PLMs should be more than enough for your application, helped very much by the 1/800s sync that you get from the Hasselblad.
I have gone through the modifier acquisition syndrome, but eventually, realized that the Maxilite, the deep octa and the PLM are my go-to mods (Or good old reflective umbrellas if I am working with speedlights and moving fast). I'd recommend the PLM to everyone. It's not an "easy" mod to use as in, you can't just pop a strobe into it and expect gorgeous light as you would with an octa. You have to understand its character a bit before you "Get" it, but once you do, every strobe based photographer will benefit from this wonderful mod.