I'm think about how it influences airflow *over* the car, not *under*
Example: my V70 was terrible for the rear screen getting covered in dirty spray, because of the vertical rear screen - there's a vortex in the airflow at the rear that effectively sucks dirty road spray onto the screen. But when I had the towbar-mounted bike-rack and 3 mountain bikes on, the rear screen stayed clean, because the bikes broke up the vortex at the back, and the spray just covered the bikes instead!
What I'm worried about is that fitting a rear spoiler to the Leon will make a bigger vortex at the rear screen, resulting in having to run the rear wiper at all times in the wet.
Hope that's clear now.