Cool Andy, good to hear they've implemented ground effect. I did my ground effect tests in a way very similar to yours, but before 4.08.
Taking into account that real ground effect is only there between 0-10m, it has no significant effect on the average sink rate measured over 500 m of altitude.
I've also experienced "the other" ground effect. I reproducibly experience it as a sudden turbulence when i pass above a coastline ingame. However, it had no effect on this test, as i flew above water at all times. I even have all tracks, really boring stuff though
