I had some fun trying to imagine what the airflow could look like with such a layout. Now, notice that i'm not saying that the following does happen to your car...i'm saying that they could happen, that there's a risk.
The transition zone into turbulent, which always appears with high angles of attack, could negatively involve the surface of the wing itself, decreasing its extraction capacity, thus downforce.
A gap feeding the rear part of the wing could solve this problem: the transition zone would still be there (as i said, it's quite inevitable with high downforce wings), but its entity would be decreased and the wing surface wouldn't be affected.
About the rear end of the car, as CAEdevice said, that diffuser is quite dramatic. I imagined two situations, shown in the following pics.
In the first one the flow doesn't separate, but after going through the first throat, it meets a "wall" where the pressure builds up (in the red circle). That transition could be more gentle. That thing may also cause the problem shown in the the other pic...
...in fact, that steep expansion followed by the previously cited "wall" could lead to a recirculation zone, which would de facto decrease the expansion area much below its potential maximum.
As always, vortex generators could make our life easier...
Last two considerations:
-front suspensions cover upwashes quite considerably: this helps front downforce but decreases the rear. In my tests, with my car, the total downforce balance was negative with this kind of solution.
-(this one is obvious...) the rear wing should be imported with more points
Thanks to Machin and to Variante.