What i've noticed, to your point, is that its not just about "staying" asleep. The "efficiency" in terms of hitting all 4 stages of sleep also improve. In other words, i get more rest for the same amount of time asleep.
It sounds like you're paying attention to a much more important kind of "sleep efficiency": rest per time asleep. A better name for Sleep Restriction Therapy's "sleep efficiency" might be "bed/sleep ratio". (Some scientists do a terrible job at naming things.) Who knows, maybe when Dee wrote "sleep efficiency", she meant rest per time asleep. (Dee? Are you still here?)
BTW, I've had occasional good results with sleep deprivation. Quite a few times, after getting no sleep for a night, or only about an hour, I've snapped out of "brain-scramble", and retained my ability to focus for several days. Not always, though. Sometimes I've just felt more drained. I've seldom done this on purpose, though, so it's hard to say what factors make a difference.
This happened two days ago, actually. Monday night, I didn't get to sleep until about 2:00 or 3:00 a.m. At about 4:00 a.m., my fire alarm started chirping every few minutes: its way of saying that it needs new batteries. I replaced the batteries and never got back to sleep. Tuesday morning, I was massively brain-scrambled, and I had a pretty full day ahead. Around 1:00 p.m., the brain-scrambled pretty much lifted. Possible confounding factors: I fasted on Monday; Tuesday was filled with more pressures and more social interaction than most days; on Saturday, I started massively increasing my intake of leafy greens and limiting my intake of carbohydrates; and on Tuesday morning, I took a hot shower instead of the cold showers that I've been taking for the last two months (because I was in a hurry!).
Have you read anything about the effects (beneficial or otherwise) of occasional harsh sleep deprivation?