Personally I'm not convinced the RS badge will do anything for the Fiesta ST. Let's face it, the FiST is pretty much the cheapest performance car in the market. The 0-60 times aren't all that impressive, so from a marketing pov, it's terrible to car to sell by the numbers. It's a hot hatch, so it's not going to stand out in any way to the "average joe" either as a "performance car". On the contrary, I've seen people who don't know anything about cars think the BRZ's a Ferrari or something. lol
And like others have said, 90% of people on the road only use their car for A to B commuting. Your Fiesta ST looks like a Fit or Prius C. Nevermind the more aggressive styling "bodykit" on the FiST, they probably wouldn't even pick up on your red calipers or your low profile tires.
As for the actual car enthusiasts... There's still a good chance they don't know much about the Fiesta ST. If you're lucky, they might think it's a GTI or something (lol, or in my case, my car enthusiast friend thought it was my winter beater when I surprised him with my FiST during the winter). It's not JDM or Euro (that's a big deal in my area), because unless you're talking muscle, no one knows anything about American performance cars.
I can only hope the ST will make a bigger name for itself, but for the time being, the FiST (and even the FoST) is as rare as a unicorn on the street.