One for the physicists -
It would seem obvious that the lower rotor in a Y6 (or any vertical rotor set-up) is working in a whole different way to any rotor in a hexa set-up. The fact that it's pushing down "pre-pushed" air suggests, intuitively, that it's not working as hard as it's upper neighbor and therefore not generating as much lift.
So is a Y6 weaker than a hexacopter, with every other spec identical?
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Just as a note on efficiency, almost all of these garden-variety model airplane propellers are dreadful at static thrust efficiency. See this linked paper and note that for low advance ratios (J), most fall below 30%. Just as a comparison, full size helicopter rotors reach 70% or more.
http://www.ae.illinois.edu/m-selig/pubs/BrandtSelig-2011-AIAA-2011-...
Thanks for you help Mike. Ill see what I can glean from these links.
Having researched this a great deal, and having tested (under admittedly sub-optimal, less-than-laboratory conditions ) a number of different rotor configurations, might I suggest a more modern paper might worth perusing. I know many aeronautical engineers believe that coaxial configurations offer promise of increased efficiencies, but I am unconvinced. Intuitively, having four or more drag-inducing blades working the same column of air just doesn't seem like it would work well (for much the same basic reasons we don't see biplanes much any more). Dr. Leishman's analysis of the available data, including review of the free vortex wake models he produced, are summarized nicely by the chart on page 9 of the attached paper. If anything, it should illustrate the fact that even acknowledged experts often can't agree on something seemingly so basic.
http://terpconnect.umd.edu/~leishman/Aero/AHS2002_Griffiths.pdf
It is the engineers, not the physicists, whom you want, I believe.
14% is mostly theoretical/optimistic for full scale, variable pitch, but larger than 33%, I think, in this case, and in typical coaxial arrangements vs similarly arranged single and tandem rotor systems.
See http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970015550_19... for a good read on a survey of the topic, across major research performed by several countries, which I believe you will find applicable.