I picked up a SRAM T3 hub from an abandoned bike and installed it on a Dahon. I've ridden coasters very briefly on holiday but never looked at the mechanism or taken on apart, until now. So I've just been examining this hub and the design strikes me as weird. The brake shoe thingy could have just a little more clearance from the shell than they seem to such that there's not a constant drag between it and the hub shell, which there seems to be. It looks as though the brake shoe thing is more held in place by the shell than by the coaster mechanism, which mainly prevents the shoe from rotating... seems like this could be designed better.

Are all coaster brakes created equal and what sort of efficiency loss ought one to expect?

Some brakes have a more positive disengagement via a spring, but the reality is that incidental skimming contact doesn't cost a meaningful amount of drag. Friction is proportional to the force holding two surfaces together. At zero force, even though there may be some contact there's zero friction. The first revolution or two of the hub pushes the shoes inward so they either don't touch at all, or just skim the highest (lowest) point as the shell spins.

So yes there's some vestigial contact and a trace of friction, but in the scheme of things it's close enough to zero that it's meaningless compared to the other causes of drag on a bicycle.

Nah, there's a couple different designs and varying quality.

Depending on how you look at things, there is a lot of variation among coaster brakes. Of course, most of those variations will seem pretty insignificant to anyone who isn't, like, really into coaster brakes...

Some coaster brakes have internal discs, while most modern ones have internal drums. Some have roller-bearing clutches (old Sachs, Velosteel) which definitely reduce drag, but I've yet to meet a coaster brake hub that didn't hae some drag, which tends to be noticeable mostly on the stand or low speeds. (When I am walking any of my bikes with the Shimano cb-e110, I can definitely feel the drag. Once I'm over prolly about 5mph, I don't feel it anymore...but it's likely still there.) The thing is, most folks who are running a c/b hub are not so worried about performance and efficiency as they are about low maintenance and simplicity, which are areas in which the C/B hub shines.