Ground effect is due to the air's inability to move downward as the wing passes, so the air underneath the wing is compressed.
High velocities over the upper surface aren't needed. The lower surface experiences laminar flow in the boundary layer. This results in a lower Cl for the same lift, and a lower Cd.
It's as if the wing's AR is increased.
The Shuttle and SR-71 land softly because of the compression of the air.
Your wings see the same thing.
The problem with exploiting the condition is turning. Any bank changes the distribution of the air pressure in an exponential manner, the lower wing seeing the most, the rising wing experiencing a large change.
This is why those planes built to use the condition have short spans. Ekranoplan, Lippisch, etc..