After some hardcore Autobahn testing I've found that my (similar) spoiler induced enough front downforce to bulge the plastic floorpan below the engine, each time I was above ca 200km/h!
I had to kneel down and push it back up, two times last eurotrip.
For you speeders I've come up with a simple solution:
1. Drill two small holes in the center of the pan, preferrably in line not to catch stuff, grass etc
2. Tie wrap the center of the floorpan to the radiator fan mount/chassie.
Thats ok!
It works, its invisible, don't add weight & is reversable, wont break if pushed - good enough for me.
Also, the skidplate is regularly taken off so it has to be something easily undone/done.
I'm sure somebody will eventually come up with a more elegant solution.
Point is; you'd prob have the same issue with the Schmiedmann spoiler.