Agreed, this sort of spoiler/wing probably won't produce much downforce but it will add a lot of drag.
No performance change will be noticed, apart from a lower top speed and a longer time to get there.
If fitting because you like the looks, then fine. If you expect an actual performance improvement you will be disappointed. However, you may experience a 'placebo' improvement, similar to that experiened by people fitting cheap induction kits.