sar593 was saying 10% loss for every 1000m (note m = metre), M5 says 3% loss every 1000 feet, so without a calculator and exact conversion ratio it seems to be about the same.
As for Superchargers losing power compared to turbocharging, a lot if it depends on the type of Supercharging the engine has, the less efficient roots type (blower), the much more efficient linear twin screw or Eaton type or the centrifugal type which is constructed like a belt driven turbocharger and behaves in a similar way too.