Aucky,
The myth that ground cushion is a pressure bubble that supports you is picturesque and actually helpful, but really physically inaccurate. At some point, it is useful to transition from the basic explanations that tend to simplify (but may work) to the actual physical relationship, which is deeper and more meaningful.
Power goes down as speed goes up beacuse the power "savings" of IGE acts only on the induced power, which drops to insignificance at mid speeds, so the effect of IGE at speed is small. This is not because the bubble is somehow left behind.
As some kind of proof, look at an airplane achieving significant ground effect at 150 knots, where the bubble is far behind the aircraft, as seen by the disturbance on the water behind the aircraft. The ground effect works even so, because the induced drag at that speed is high, so the 15% savings is significant.
An Ekranoplane showing the "bubble" behind the aircraft.
