From an economic standpoint, I can't see an argument for buying a FoRS right now unless you have money to burn. If you think about it like a stock, it would be like buying at the peak. The RS is "so hot right now" and there are relatively few of them (as Ford intended), so dealers are happy to charge a markup for them. And they are still selling pretty well:
http://www.detroitnews.com/story/business/autos/2017/05/08/hot-cars-selling/101454922/
So you have two really compelling reasons to wait in my opinion:
1) Wait for the price to come down on new models or for more of an affordable used market to develop.
2) Waiting to see what the fatal flaw is
By fatal flaw, I mean on a car this high strung, with Ford's reputation (or lack thereof) for build quality, something is going to break, and probably in significant numbers. It may be something on the engine, or something in their trick AWD transmission, or something else entirely. Until it's been out in the world for a few years with people beating on it, you won't know what that is or how much it is going to cost. Even if you are comfortable with whatever build quality/maintenance issue arises, better to buy after it is known to the world than before, so the price can adjust accordingly.