I've actually tested vortex generators on a Clio Sport. They don't work. The company selling them guaranteed that they worked on all body types. But their own explanation of HOW they worked gave me doubts, so I got the guarantee in writing, bought the product, tested them, found they didn't work, and sent them back and got a refund.
What vortex generators do is prevent air separating from the surface at a curve. So on the Evo vortex generators above the rear window means that the air flows down over the rear window instead of separating turbulently, which causes drag. Having the air follow the rear window also means the rear spoiler works better.
But a shape like the Clio Sport doesn't have a sloping rear. It deliberately has the "spoiler" above that stupid curved rear window so the air doesn't try to follow the curve of the window. The spoiler causes clean separation that doesn't cause drag.
So, yes, vortex generators do what they are designed to do. But the shape of the Clio Sport is not one where you want to do that. You don't want the air to try to follow the body shape. You want clean separation at the top of the rear window. That's what the standard spoiler is there to do. Not generate downforce, but to get clean separation.
