When i saw all the issues that dogged buyers of the Focus RS, i was not surprised to see their discussion forums go totally silent regarding anyone modifying those engines. All the enthusiasm evaporated once the head gaskets failed (incorrect part installed at assembly)
And no, Ford would never give the ST that much power off the floor. They'd have to give the Focus ST 300hp to compensate, and the RS 400hp. That would beat the Mustang. Hard no.
They can be safely tuned to those numbers, but they're sold to the public.
The turbos are the strongest engines in the platform. They are built to handle more power than stock, and they last.
Ford has stated that the peak H.P. number they assigned to the Fiesta ST is the momentary overboost figure. So, in stock, real world, even fairly aggressive "normal use" we don't use or feel that exact number. Im ok with that.
I must assume that H.P. calc' method holds true for the Focus ST and RS.
The Focus ST has a history of valves getting coke accumulation that cause misfire or failure to run.
The Focus RS has been a heartbreaking nightmare of unreliability. First issue being the incorrect head gasket on some the second being the gear oil leakage from either the transaxle or transfer case. Both of those issues caused the unfortunate buyers to lose the use of the cars, that in most cases ,they paid well above list price for.
In the case of the RS it caused a chilling effect on owners doing any mechanical modifications.
Their forums are mostly silent. Two guys that work in the same complex as myself, took delivery of their RS's, a blue one and a black one. Both head gaskets failed within a month. The blue car began leaking fluid right after it returned from getting the head gasket.
After a long period of time, both guys used the lemon law to walk away from them.
I don't feel good about relating that story. I wouldn't want to have it happen to me.