Crab,
This is, like many pprune threads, a really good one, and an eye opener. We are then left with this paradox:
The pressure under the disk IGE is the same as the pressure under the disk OGE. This means that IGE does not have a "ground cushion" of pressure. There is NO ground cushion, no pressure increase. The change in lift is due to the earth creating, in effect, end plates that block the sidewash of the airfoils.
OED
BTW, the downward flexing of the altimeter during the pitch pull is due to the static ports getting bathed in the downwash of the rotor, not a general pressure increase. Once the flow becomes steady state, the altimeter is a good indicator of the pressure under the disk, and when it shows no significant difference from the field elevation, it tends to prove the non-pressure case. I did some number crunching, for a 15% drop in power to be caused by pressure alone, the pressure would have to change about -150 feet's worth of altitude.
Note that real GEM's do create a pressure below the aircraft, and they have a skirt to allow the pressure to build. A manometer that reads the pressure under the skirt would show a significant pressure rise. Helos in ground effect are not the same beast at all.