When I worked at JDM we had multiple customer cars in the low 10s and high 9s and the shop car running 9s brackets and low 9s in exhibition with no SFCs. stock style rear
suspension. I've also personally driven multiple S197s on-track (road course) and not felt the need for them. A good set of sway bars and springs does a lot for these cars on-track IMO.
I have through-floors that tie to seat braces, the rocker panels, and Wild Rides lower s-boxes in my 98 Cobra, but that is a significant amount of work.