Regarding efficiency. If you compare a single stage and two stage compressor that pulls 30 amps, it doesn't appear that the two stage flows more air @90psi.
So are you saying the efficiencies accrue at higher pressures? And if so, does this matter if you never use more than 90psi? (i.e. a car with only one gear is fine if the roads are so bad you can only drive 10mph.)
TubeTek makes an interesting point regarding thermodynamic law. Energy cannot be lost--put in 30 amps of electricity and you get 30 amps worth of air, vibration, and heat, right? That means a more efficient compressor would have to produce less of one or both of those unwanted byproducts.
I've never been able to wrap my mind around why two stage compressors are better. Having said, that, there are a lot of things I can't wrap my mind around