Should not be
The st170 bottom arms are the same as the RS ones, not sure what the difference is between the ST and standard focus but they are all the same shape and have the same ball joint. The real difference is the mounting to the sub frame a standard focus uses a 12mm bolt on the front where as the RS uses 14mm (not sure of the ST) and I am guessing that the ST has an uprated rear mount and the RS one was camber adjusted(could be wrong!!)

So fitting a ST170 sub frame and arms would be a good start and if it uses the smaller bolt at the front it would be easy enough to upgrade to the larger one and fit an upgraded poly rear mount if required, hope this helps

anyway apart from the delivery of the RS gearbox no real progress is being made on any of the cars sorry to report