Ken, I couldn't tell you
and the nature of the beast makes it feel a little moot to me - if you measured directly, you'd see a drop of at least a stop I'm sure, but then you get more spread-out, soft light in the overall scene. Plus, I can't remember a time when on-camera flash didn't have enough oomph for me, even in the film days, shooting EPP at 80 iso. But for, say, a wedding guy or someone who does a ton of events, I'm sure there's a more thought-out approach. I use this for meet & greet, minor celebs in town kind gigs, and I'll likely whip it out at a party or two - all digital though.
It's essentially like those $60 Gary Phong tupperware-looking things, just a lot cheaper and seems to spread the light much more. It is the same all around
I've thought about putting foil against the back interior of it, but I don't do enough on-camera stuff to need to play with it too much. $19 and Amazon's return policies, maybe worth a try if you're not psyched with your current approach. I used a lumiquest reflector-thing and a rubber band for the last 15 years, so for me, huge improvement.
Just gutted a Speedotron head to put my Speedo reflectors on a Vivitar 285 - MUCH more excited about that, with a radio slave it's a cheap & portable deal for more of a beauty dish look, but I'm sure it could take a good sized strip box or softbox.