Long term project and work in progress. Won’t really know if there’s any benefits on track but will help cooling.
This looks slick! Are you running a flat floor as well?
For me, I think the benefit of adding a splitter and flat floor is not so much to increase downforce, but to reduce drag. The underside of a road car is a mess with flow separation all over the place which slows down the speed of the air passing underneath the car increasing drag. The problem you then run into is engine bay cooling, your only choice really being venting out of the bonnet, or ducting your radiator out somewhere. Any diffuser you would add, maximum angle you want is about 12 degrees before you start getting flow separation.
In terms of actually creating downforce, as other posters have pointed out, at the speed our Clio's can travel, and the ride heights you can realistically achieve, there isn't much you can do. That being said, dive planes/canards at the front and a gurney flap added to the rear - I think - would give you a minor increase in downforce for a slight gain in drag.
Kyle Drives on youtube has some great videos about the basics of aero, with CFD examples to show pressure spots and air velocity across the surfaces.