I finally had the time to confirm and document what I've known for a while with regard to using Speedlight's with lightboxes.
This is a real Mythbuster, folks! 
Many people ask about using softboxes with Speedlight's and a huge concern seems to be whether the softbox is too large or if it's going to eat up too much power.
Typically, the person asking the questions is dissuaded and smaller modifiers are recommended. That's a shame. The misconception is all over the internet and it's time to put an end to it! 
Rather than continue this guessing game I set up a small selection of modifiers ranging in size and type, and took a meter reading. The numbers speak for themselves and the bottom line is that you can pretty much use whatever size or shape modifier you want without sacrificing anything.
I took all readings with a Sekonic L-758DR mounted on a light stand in a fixed position, 6 feet from the face of the modifier, and set to meter at ISO 100. The flash is a Nikon SB-900, which I believe has a slightly lower guide number than a Canon 580EXII. Flash zoom was set to 24mm and power was at full (1/1). The flash head was rotated 90 degrees so the orientation matched that of a rectangular softbox in vertical orientation.
I went from one extreme to the other, starting with a small 16" x 22" softbox with a single layer of diffusion, all the way up to a 54" x 72" softbox with an inner and outer layer of diffusion. I also threw in a few umbrellas just for comparison.
And here are the numbers.
46" Umbrella - white satin interior - black backing - reflective bounce - f/8 + .3
46" Softlighter II - with diffusion panel - f/5.6 + .7
43" Umbrella Softbox - f/8 + .5
16" x 22" Softbox - white interior - with single layer of diffusion - f/8 + .8
24" x 32" Softbox - white interior - with inner and outer diffusion panels - f/8 + .4
36" x 48" Softbox - white interior - with inner and outer diffusion panels - f/8 + .7
54" x 72" Softbox - silver interior - with inner and outer diffusion panels - f/8 + .2
Since those are readings from the center hot spot I also went ahead and took some additional readings with the 54" x 72" softbox to see how well the light was being distributed and what the coverage was like. I found no more than -.4 EV difference from center to the short edge. It was about -.2 EV from center to long edge. Very respectable and as good as with some modifiers used with studio strobes.
I was a bit surprised by the lower reading from the Softlighter II but that does explain why I too believed that umbrellas and indirect umbrella softboes were not as efficient as enclosed lightboxes. I was also surprised to see the Cheetah 43" umbrella softbox with a higher reading than the Softlighter. 8/10 stop higher, to be exact.
You can see that going from a tiny softbox to a very large one only shows 6/10 stop difference and still gives a very high output. It's not losing multiple stops of light as many seem to think.
So, in terms of output, battery life and recycle time, it's really all the same with very little difference and no reason to worry about modifier size unless your flash does not allow for wide angle zoom settings such as 24mm. I didn't go crazy changing the zoom setting but I'm sure that you can optimize things by doing tests to see which zoom setting is best for each size modifier. I figured this would be a good general reference.
