Juicy, the single stack suffers more from cool down by default, thats just physics, since the volume of gas reservoir is a lot less, each shot reduces the pressure of the gas reservoir by a greater percentage, and the temperature is dropped by the same ratio(
http://en.wikipedia.org/wiki/Ideal_gas_law). And as it gets cooler it get less efficient again as the gas liquify.
In general with lighter moving part(stock/light weight BBU), lighter rate springs(stock), tighter seal piston(something with O-ring), tighter seal flute valve, tighter hop-up seal, and stock mag valve, that'll probably give you the best gas mileage with least consumption....also, using lower pressure gas helps, at the expense of fps.
Slono, that sounds more like a seal-related problem, I am guessing something to do with the floating nozzle...though not sure how....