Yes, I painted the firewall on my RV-6. FWIW, I polished the stainless on my Rocket. I scuffed the stainless and used a two-part epoxy primer and DuPont single stage polyurethane white paint. After three years it still looked new. Poly paint is good to 450 dF continuous and 500 dF max, well within the temps inside the cowl.
Thanks for the responses. I had seen some firewalls painted and was not sure if there was any benefit. Does not sound like there would be. Good thing, because I hate painting