Following up a year later, looks like the more common Chinese wheelsets (for ex. from Yoeleo) come in the usual depths but with three different widths now. Width is defined here, for tubular rims, as width at the brake track (BT):
Depth (mm)x BT Width (mm)
38mm x 20.5 / 23 / 25
50mm x 20.5 / 23 / 25
60mm x 20.5 / 23 / 25
88mm x 20.5 / 23 / 25
Issue 1: rim width variants.
As far as I know, most 20.5 widths are the older, more 'V' shape profile, while most of the 23mm or 25mm widths are now more 'U' shape. They don't appear to be as advanced as a Zipp Firecrest shape because they seem (from photos and videos) to be more narrow at the spoke hole-side and less rounded. The 23mm and 25mm versions do often bulge out from the brake track, however, giving us two width metrics when viewed in profile: brake track width and bulge width. For example, the 50mmx25mm Yoeleos have a bulge width of 26.27mm. This is the shape most of us want, the one that is closest to a Firecrest (which I'm placing as the industry's most advanced standard). Short (generalized) story: the wider more bulging U-shape, the more aero for most types of riding (as of early 2014).
Issue 2: tire width + rim width aerodynamics
As many on here point out, you should also consider your tire width when choosing rim width. Wide tires on narrow rims aren't as fast (the theory goes) than matching widths more closely. Lots of variables in there, but the numbers on our gold standard 404 Firecrests, again, show them being best with 23mm tires. Short (generalized) story: match your tire width to your rim's brake track width.
Issue 3: rim bed design
Lots of mis-information and guessing about this, but for the most part people (amateurs in online forums, like us) seem to think rim width doesn't matter to tubular glueing. I'm not convinced. I think more clarity is needed when discussing this factor: rim bed profile matters more than rim width. You can have a 25mm rim width that is better designed for 23mm tires (aka Zipps). I'm guessing some 25mm rims do not have such an optimal rim bed. The main concern I have is A) having a tubular tire be less secure because of less contact on a rim bed and B) rim bed design negatively deforms the shape of the tubular tire affecting rolling resistance or cornering ability.
I'd like to know if the base tape on a narrow tubular tire might not fully contact the rim bed on wide rim. Example: a 23mm tire on a 25mm rim. Differing opinions, but I'm hesitant to go that route, and will likely go 23mm tire on a 23mm U-shape rim. Even found a comment direct from Yoeleo about their rims and that combo:
"The 25mm wide wheels are U shape rims, please use the 25mm tire or 28mm tires, 23mm tire can not fit well.and we will able to offer the 23mm wide u shape carbon wheels in 60mm and 88mm depth this month [Sept. 2013]" — Source (in comments below video): http://www.youtube.com/watch?v=x6BbopKHnf0
What they mean by "can not fit well" isn't 100% clear. 23mm tires don't glue well to that rim? Or the tubular tire doesn't shape well? Or the aero properties don't match well? I'm guessing the first option because I don't think Yoeleo is that precise with their aerodynamics or care that much about tire shape, but they may have tested a few glued tires and felt it wasn't glueing strong enough.
As for mismatched rim beds deforming tubular tire shape, I'm less concerned but looking for a more expert opinion or examples.