Maybe you could create a spoiler like doc Gerold (no that's not me, I am Ger
relt..
) :
But then with some slots in the spoiler near the top of the rear window to let some air through for the intercooler. I think such a spoiler is aesthetically pleasing too.
I can also send you one of my spoilers, then you can try if it makes any difference. If it doesn't work you can send it back.
But I think at such speeds you need a bigger spoiler then mine.