Lightweight rims will make a big difference. But, there's another way to approach your rim decision based on the SCCA Category you are running in, and looking at the car as a whole.
I use SVE S350 18x10 rims (front and rear) on my CAM-T New Edge cars, they weigh about 24 lbs. each. The 18x9 S350 rims, which are easier to fit square on 94-04 Mustangs, weigh 22 lbs. each. Tire Rack shows the 18x9 RPF1 rims at 18 lbs. each. The SVE S350 rims cost $200 each; the RPF1 rims cost $380 each. That's $720 more for 4 rims. That extra money could buy you tubular A-arms and lightweight bumper supports, or other lightweight parts to help with mass centralization and handling. BMR 94-04 spring perch A-arms take 6 lbs. off the front end and improve handling; a lightweight front bumper support can take 16 lbs. off the front end; a lightweight rear bumper support can take 21 lbs. off the rear end. For the same money, you would end up with a lighter, better balanced car. It's worth considering your options, especially if you're in CAM.
I have used SVE rims for years and they have held up well under autocross conditions. I have videos about choosing the best autocross rim and tire size, fitting 315 tires square on a New Edge Mustang, and weight reduction/balance on my Warhorse Racing YouTube channel.
If you ever have any questions about autocrossing your Mustang, please ask. I'm happy to help.