A couple of weeks ago, someone posted a photo of a BRZ with a spoiler installed backwards. We all laughed, but I was curious what the difference in forces would be when installed properly. So I made this obnoxious ricer spoiler in Solidworks and tried to determine some forces at 100 km/h.
Installed properly, the force in the Y direction was 315N downward, with 123N of drag in the Z direction. Backward, that becomes 55N upward (Y) and 155N of drag (Z).
So that's interesting, but not surprising if you understand a small amount of physics. Probably won't want to hit the track with it backwards, anyways. Obviously this is not particularly to spec (it's approximately the width of the BRZ) so take the numbers with a bucket of salt. I'd be curious to see how that changes with proper dimensions and shape, too.