John, I think your analysis makes lots of sense for a gravity fed fuel system (i.e. no pumps).  However, I would think that the EFI pumps would churn the water and fuel together pretty thoroughly. 

Yes the pump will mix it up pretty well; but it will separate out again in the line..

In any case, I did use some auto gas back a couple of flights ago and did not like it.  The engine had a bit of a miss – nothing bad, but just noticeable – could have been the plugs needing a change to be fair

It seems to me this is unlikely an effect of using mogas.  Running my engine on the dyno showed a very slight improvement in performance with 87 octane mogas over 100LL.  I think the theory was faster propagation of the flame front in the odd-shaped combustion chamber.  And as you know, the lead is no friend to the spark plugs.

Al