My .02
We are too far along in the current production cycle of Fiesta for them to be doing something crazy like doing the R&D work on a possible production Fiesta RS. With the plans to kill the current design and go forward with a newer design somewhere in the next 2-3 years, it would be a poor business move on their part (although it'd be totally amazing for a one-year production run for us car guys.....just think of the appreciation!)
With that being said, it would also be pretty hard for the FiRS to NOT step on the toes of the FoRS. With us having the lighter chassis and, in my opinion, better handling chassis, the Fiesta would grab all those potential Focus owners who want essentially the same thing, but in a lighter package.
Then there is a third point, being that the Fiesta is simply not nearly as numerous as the Focus (from what I have seen....I don't know the actual production numbers). When I lived out in San Diego, I could come across another FiST at least once a week and the same applies for the FoST. However, after moving to Chicago I have yet to see one FiST here, while I have seen a plethora of FoSTs.
The good news is, if we are in that much smaller production numbers, keep your car rust-free and well-maintained and you should actually see some appreciation in value a number of years down the line.