Upload & Sell: Off
I did some tests. 24mm seemed to fill it just fine and there wasn't as much spill around the edges as I suspected.
Using the flip-down diffuser, however, made the light MUCH more even. No scientific measurements, or anything, but you could see it eating some of the light. I think for most cases I'll have plenty enough light, so I can probably afford to use the diffuser to even it out.
I spent some time with all of my stuff mounted on stands and my camera on a tripod to take matching photos showing all settings. I had 3 series - One showing fill directly; one showing fill, spill, and light quality on wall; and one showing head on to see diffusion in the umbrella. I wanted to post the results here, but I didn't have time then lost interest. This is why I don't blog. Sorry, everyone.