I added a high wing, front and side skirts, rear diffuser with canards and YES it makes a huge difference.
One thing I've noticed is that if you add a wing, there will be so much extra downforce that you'll notice that your rear hatch will begin to sit noticeably lower and you'll need to compensate for this.
Unless you do a lot of drag racing, too much downforce isn't a real big deal, although it does slow the car a bit, but not that much and you will make it back in the corners because you won't need to slow down as much.
Corners and apexes are where the carbon additions really kick in.
The car is far more stable and adheres to the asphalt much better increasing traction and enabling the driver to maintain greater speeds with less wear and tear on the brakes.