There are three studies that I know of that have reported very high efficiencies, all in elite riders: Lucia et al, Santalla et al and Sallet et al. If Andy C knows others, he’s welcome to post links and I’ll add them to the discussion. Let’s assume all the subjects in these studies could put out 90% of their V02max for an extended period of time. How many would be capable of > 6.0 W/kg.?
Lucia study. From the graph posted upthread, one can get V02max and GE values for individual riders. It appears that only 2/11 riders in this study would be > 6.0 W/kg, and they had very high efficiencies, 27-28%. The highest V02max values in the group were 82-83, but with GE values that indicate only about 5.6 W/kg.
Santalla study: Only DE values were measured, but it appears that only 1/12 subjects might be in the 6.0 W/kg range. This rider had a V02max of about 80 with a DE of 30%. He would need a GE of about 25.5%. There were three V02max values of about 85 or higher, but again, they were associated with GE values that would indicate significantly less power than 6.0 W/kg.
Sallet study. I can’t access the full paper and get individual values, if they report them, but they reported a mean V02max of climbers in their study of 78.2 +/- 5.5, and the mean GE was 25.6 +/- 2.6% for the professional group. A combination of the two means implies a power value just below 6.0 W/kg. There were 24 climbers in the study, but I don’t know how many were in the professional group, which had a higher mean efficiency. They didn’t report an inverse relationship between V02max and efficiency, though, so it would appear there would be several above 6.0 W/kg. If we assume half the 24 climbers were in the pro group, half had a V02max above the mean, and half had a GE above the mean, that would be three. There might be one or two in the amateur group as well.
So speaking very roughly, about 10% of the subjects in these studies might be over 6.0 W/kg. I want to emphasize again, though, that this assumes 90% utilization, which presumably is not a 100% certainty, but would need to be factored into the probability. Also, the studies are not strictly comparable, because the kind of riders studied might not be quite the same (the Santalla study looked at the crème de la crème, and the Sallet study isolated climbers as a group, while Lucia AFAIK did not select just climbers or GT specialists). And finally, the Sallet study apparently found no inverse relationship.
Another relevant figure we can glean from these studies is the proportion of riders with high V02 max values. In the Lucia study, 2/11 > 80,and none > 85; in the Santalla study, it was 3/12 > 80, with 2 > 85, and the other just about at that value. In the study by Moseley et al., which also examined elite riders, but found much lower efficiencies, it was 2-3/16 > 80, none > 85. So—again bearing in mind some differences in the composition of the subjects—it seems that very roughly about 20% of these elite riders had a V02max > 80, < 10% > 85, and none > 90.
A final point I want to make is the possible effect of doping. Andy Coggan, I think it was, argued that if these riders were blood doping at some point relevant to when they were tested, it would be reflected in higher V02max values, but not in higher efficiencies. And in the Santalla study, which reported an increase in efficiencies over a four year period, there was no change at all in V02max.
However, the situation is potentially more complex. Acute blood doping—EPO and/or transfusions—would be expected to raise V02max, but the effect would be reversed when the doping stopped. So if, e.g., the Santalla study measured these riders at the end of the season or in the off-season, when they probably would not be doping, one would not expect to see a change in V02max.
The same is not necessarily true for efficiency, though. Though EPO is known to cycling fans as the hormone that increases the synthesis of red blood cell precursors and thus HT, it has many other effects and potential effects, some well documented, some controversial. Particularly relevant to this discussion, EPO is well known to promote angiogenesis, or the formation of new blood vessels, and some studies have shown that it increases proliferation and enzymatic activity of mitochondria. All of these effects are candidates as possible processes underlying increases in efficiency. The mitochondrial effects are fairly well documented in heart muscle—where they could potentially augment the effects of EPO in increasing HT by increasing blood flow and V02max—but some studies have also claimed such effects in skeletal muscle, along with increases in size of the muscle, where they might show up as increases in efficiency, not in V02max. The best evidence comes from studies of diseased patients, rather than healthy ones, who might not seem to be very relevant to elite riders, but keep in mind that pro cyclists operate on the edge of metabolic breakdown, and in some respects really are not like healthy controls.
The evidence that EPO can enhance skeletal muscle function is controversial; there’s certainly no consensus on this yet. But assuming that EPO might have these effects—very possibly differing markedly in degree from one rider to the next—one might expect them to be more stable than the effects on HT. That is, increases in mitochondrial number and/or activity, or muscle fiber size, would probably be somewhat more enduring than effects on HT. So it’s at least conceivable that if riders were taking EPO for significant period of time every year, they could enhance their muscle function semi-permanently, so that it would result in measurable changes even when they were not taking the drug.
Again, this is all speculation, but certainly something to bear in mind. Here's a recent review of the subject:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3710958/
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