Without coming across as insulting, I think we are agreeing in a circular manner with slightly different details.
In theory long as the system stays sealed and uses a recirculating bov setup, then the pressure stays an even constant. But both superchargers and turbochargers compress air the same way and as long as its a sealed system they work the same way. They both have the same effects at negating altitude for an internal combustion motor simply because they are forcing more air into the same volume, essentially making the thinner air at altitude a non issue and keeping the power output of the motor in a range of normalcy.
The reason for two-step superchargers was to prevent over boosting at low altitudes since boost is mechanically controlled through gear ratio as opposed to pressure release valves.
Roughly 10psi at sea level is the same as 30psi at 30,000 feet. Pretty good section on altitude effects.
Again, I think we are agreed, just having a discussion on the topic.
http://en.m.wikipedia.org/wiki/Supercharger
Since the E-charger doesn't have a bov or anything, once the air is compressed and blown towards the TB, it will stay the same pressure even on throttle let off. The issue you run into then is compressor back surge, which is a whole separate issue, but I think since the charger disengages when not powered, I assume the air pressure will just back itself into the airbox through the compressor.