Neuro, as always, the math is appreciated, if not fully understood.
Even using your suggestions for modifier sizes, I'd like to emphasize that, as wonderful as the 600ex-rt is for many situations where super-portable lighting is needed, once the modifier goes on, or bounce is involved, we are looking at usable max f/stops of about f/5.6 - f/8, which is fine for some situations, but very limiting when fighting ambient light. Once you start raising the ISO, ambient light is just part of the picture.
And with the modifiers, even the nicely priced 24"x24" Impact softbox requires very close placement to the subject. Get much bigger than that, I imagine, and keeping the modifier out of the frame becomes a real challenge. (Note there is a nice grid available for this Impact.)
While the 600-ex-rt and the right little softbox can produce excellent headshots, once you want more, say even 3/4 body shots, things get difficult.
And don't overlook umbrellas for the Speedlites. I think they are even easier to use and can work great for individuals AND groups. With three Speedlites and umbrellas, I was able to nicely light a very big group of about 100 (four layers deep by about 25 people across). I think I was using 43" umbrellas, silver linings. This was early morning in deep shade. I had my ISO up a bit, but the dark trees behind the group made things easy. All I had to do in post was dodge a few faces about 1/3 stop.
I can vouch for the Einsteins. When I got my first one, I thought it would be a step up in complexity, but soon I learned how easy perfect exposures were with the CyberCommander's flash meter, and how classic portrait lighting was suddenly possible. The Vagabond battery pack is easy to hang on a light-stand under the Einstein. And the Buff modifiers are very good too.
As noted, incredible value for the money.
So, for environmental portraits, a couple or three Speedlites can add polish or a bit of drama, or open up shadows. But for isolating the subject and reaching a new level of creative (and classic) portraiture, strobes are the next step. Also, remember that composition will be more affected by the Speedlites, as you will have to frame more tightly to avoid including your lights and stands.
Plus, the CyberCommander with the right trigger works with Speedlites too, so you can easily combine the strobes and the Speedlites. Say you use one Einstein for lighting the subject, you can put a Speedlite or two in the background to add depth to the image (once you find the right lighting ratio, remembering to keep the f/stop down a bit on the subject so the Speedlites provide the effects you are after).
Lighting! One of my favorite topics!
