18” can work if the pad depth & spoke design is done to clear the fronts.
When I was track wheel shopping, BC forged actually told me they could make me a custom cut 18 that would fit.
I ended up with 19” advans which I sort of regret. Don’t get me wrong, the rim is fine but track tire size availability is not nearly as good as 18”. So many good tires that I wish to run but either too skinny or too tall.
Most pre-made 18” rims are not gonna fit. If you look on advan or Mackin sites, they don’t list 18 for rcf fitment.
If budget not an issues, personally I’d run a custom cut Forge Line rim. But they’re really $$$$.

Thank you for all of the input so far. Will be sending her back to the well to pick some other mfg's wheel styles. I have final pick on wheel, but she has first pick on design. 19's it is. While I agree sport cups are a bit excessive for autocross, she really likes the way they work and I’m looking to move her up to the big road courses soon, so I’ll live with the expense of changing regularly. We're currently getting about three-four weekends out of a set.
Again, thanks for the input, still looking for the final solution.
Ill post pix once mounted and ready.

If you’re not in a rush, I’m planning to get Titan7 that are coming out around Sept 2018. They’re real forged, light, cheaper than JDM, better than Vossen Flow/Forge and in SoCal. Your girl picks the design but you have the final say? You mostly auto cross or track? Which track do you go?

I’ve had both, 20s and 19s and I’m on aftermarket 19s at the moment. 20s are heavy, and the car feels odd. If you’re not fully braced like me underneath and have all the suspension goodies to support it, the ride sucks. I went from adding 20s with Penske and usrs as my mods. Car was sluggish and lacked response. Added TOMS full under bracing, and the whole car didn’t rattle and made the ride a lot stiffer. The 20s were harsh on a bump or uneven road. Added motor mounts, and lightweight 19s ( wed sport sa15r ) paired with lighter rotors. The car has completely changed. Super quick and crisp response, turnin is slightly that of understeer, corners are fun vs worry some. Overall, from the 20s to the 19s I shaved almost 20lbs a corner on the wheel and tire setup. Add those light weight rotors, the 20lbs becomes 37lbs a corner! Again, going into a goal of max performance, I’d suggest following a similar mod path. 6 tracks and counting in The RCF.

Good luck in your selection. My vote would be 19s. While the look of 20s are amazing, you just take too much out of the car.

Those sound like an interesting option.
Next autocross is in September so that may be tight, I’m pretty sure she wants them on for the next run.
Mainly autocross, but building her up for the bigger road courses. Fontana is a favorite track, willow springs big willow is one of my favorites but I want to start her on the streets of willow. The rovsl at Fontana is like home, so I tend to run better there. She’s been building confidence there as well, over 140 into turn one with only minor pucker. So now time to take her to different tracks.
It’s her car, so yes she has total control over style. I’m ocd, so she trusts my final choice of manufacturer and build.