to actually increase performance on a race track, there is more to adding a rear wing than simply adding a rear wing. You need to pay attention to overall aero balance. If you plan on adding a wing to the rear, you need to think about what type of aero you will be adding to the front to create the proper balance.
Aero is hard to discuss due to lack of factual information, but there are a few known combinations that are agreed upon:
1. Factory BMW GTS front splitter and GTS rear wing (buy replicas, they are all mostly the same)
2. Factory BMW GT4 front splitter (and canards) and GT4 rear wing (buy replicas, they
are all mostly the same)
3. Bimmerworld front splitter, canards, rear wing. More agressive than factory BMW aero above.
4. Zebulon front splitter/rear wing combo for ultimate downforce
Here are two threads that are worth a read that might explain what you want to know. For good replica GTS or GT4 rear wings, I recommend Ridgeline Motorsport. They have some good quality stuff (I have their front GT4 splitter).
https://www.m3post.com/forums/showthread.php?t=2066989
https://www.m3post.com/forums/showthread.php?t=1794400