Depends on what your budget is.... A good wing is going to be 2-5 grand for the foil itself.
For whatever reason, there are still no well designed chinese knock-offs. I like APR as much as the next for certain things (not sure if they're chinese, but along those lines). They are certainly cheap (and HEAVY), but those APR 200 (new and old) wings are really tough to love and easy to loathe . Their bigger series wings are far --"less bad" IMO... choosing my words carefully there
So if a builder has a "reasonable" budget to dedicate to aero (few grand at least), then it's worth chatting about...
For the purposes of closed wheel GT cars like ours, I disagree about about the GF being a bandaid. Relatively dirty air (compared to a plane) with mess of turbulance from other cars and the body itself and at low speeds (car speeds), the GF has been proven to help prevent air separation and improve the efficiency on even the best designed profiles with minimal drag penalty. There are some nice NACA and Selig profiles with a high lift and low reynolds number that are very effective and still benefit from a GF at low speeds (car speeds). Many a "white paper" has been written by experts in the field on this...
My wing has a removable GF in a channel so the GF can be easily "tuned" or changed or deleted all together. The profile is a (Selig) S1223 which is proven to be VERY efficient in low speeds (car speeds)...
-Phil