It won't do squat except look good and scrape. A splitter that creates downforce needs to be STRONG. The splitters that you see on race cars you can literally stand on because they need to take that much force. These are just screwed in and are 100% aesthetic.
Sadly, this is the truth.
Aero appendages on cars used at highway speeds (say, 80mph or below) are nothing more than jewelry.
People will claim otherwise, but you will note that mostly the people that claim otherwise are the people trying to sell the piece of aero equipment.
Yes, some CD (coefficient of drag) numbers can be slightly altered by the addition of a small lip spoiler to the rear or a blade type front "splitter", but those numbers are negligible, at best, particularly in regular street use where our average speeds end up being under 30mph where there is no aero loading whatsoever.
On a race track, in high speed corners or on long straights, a properly designed aero piece can help some, but it takes a skilled driver, using the car at the limits of its adhesion, to notice.
It's fashionable now to have a rear "diffuser". Hell, even the ST has what looks like a diffuser panel. In reality, it's nothing more than bumper warts. It's a styling exercise alone.
With all that said, I think that black piece shown above on the white car and the carbon fiber side skirt pieces I've seen on other STs look great. If everyone would just admit it's fashion and not a "mod", then everyone would be more honest about everything.
I saw on another Forum where some guy was saying he is desperate to get his rear diffuser for his car because he can tell a difference on the entrance ramp in the morning. He's delusional or a liar. Take your pick.