From http://www.nutritionandmetabolism.com/content/1/1/2, "Ketogenic diets and physical performance" by Stephen D Phinney in Nutrition & Metabolism 2004, 1:2.
"However the interpretation of this endurance test is confounded by the fact that the oxygen cost (ie, energy cost) of the treadmill exercise had significantly decreased following the weight loss, and this occurred despite the subjects being made to carry a backpack loaded to bring them back to their initial exercise test weight."
The fact that it occured desipite the backpacks means that it wasn't mass itself, which suggests that it was either (1) lower body mass, meaning that uninvolved tissues required significantly less oxygen, or (2) improved metabolic efficiency, or possibly (3) a combination and/or some other cause.
The fact that the oxygen cost was significantly reduced suggests that it wasn't #1, but more likely #2 -- that the energy cost, in ketosis, of endurance exercise, is lower than the energy cost of the same exercise while on a high-carb diet. This tends to disagree with conventional wisdom and specific research into the oxygen cost of the two. Which is why they then followed up with trained cyclists, and used a constant-weight (eucaloric) diet instead of a weight-loss diet. This test showed no drop in performance.
The trivial way to interpret the two tests is that it was either the lost weight itself that reduced oxygen cost, or improved efficiency in non-athletes asked to perform an endurance test once a week. The latter is the easiest to believe.
Yet I wonder.... Time for me to chase down more research.
BTW, I'm moving to a different blogging platform, hence the lack of updates here. That process isn't going too speedily. :]