Whoopsy:

It's a vent, not a gaping opening that ingest air. Not a drag inducing component. Vented air merges with the air flow that goes around the side windows and back. 

Misha could be correct that the vent produced too much downforce for the Ring. But then again it's sort of hard to believe when Porsche have had vents for like 6 years now and you'd think they would have solved that problem by now.

They also been running the car at Weissach for close to 4 years now. tens of thousands of test kms. 

I suspect they have a different design for the vent pattern that they are covering up. 

Something else, could they have put the Cup car front end to the GT4RS 

 

I beg to differ Whoopsy. If you look at a picture of the 919 Evo you will see that the fender ducts (whole) are removed. Also most air vents in at the front of the car. This was done to decrease drag. Any air that goes through the body of the car produces drag.