If there was indeed turbulence generated upstream of the fin (which it may well be), then the presence of the fin will reduce it or at least the longer wavelength parts of it. What I want to argue is that the airbox was designed as streamlined even in the original configuration. The wake, save for the turbulent boundary layer leaving the trailing edge, was likely quite negligible, therefore extending the fin wouldn't help much in terms of turbulence reduction and will probably introduce additional turbulence simply due to the longer turbulent layer flow in the streamwise direcion.
I therefore believe that the rear wing actually has to work in a condition of somewhat increased turbulence, which is all the more accentuated due to the fact that the turbulent wake now has a shorter path on which to relax before hitting the wing. But this is more than offset by the reduced yaw sensitivity of the whole assembly, something which drastically improves driveability of the cars without TC.