Releasing pressure from the front fenders doesn't increase downforce, it reduces lift if we're going to get technical.
This could go off into a huge aerodynamic tangent that nobody probably really cares about but you definitely want some sort of gate or "guide" to channel air down the side of your car. Not to mention a gate will hide the hideously chipped paint that will inevitably happen on the cut-in area from being pelted with stuff that the tire picks up. Plus it will look better IMO.
I take it that you have to completely remove your fender liners for these as well? Or can they be modified to work?