19" x 8.5" wide
5-lug, 108 mm bolt circle
60 mm offset
I know that the 16" and 17" wheels use steering rack BV6Z-3504-DE
18" wheels use steering rack BV6Z-3504-CE
I am just guessing but I assume it is to limit travel and avoid tire rub issues. With the stock SE steering rack, you may have tire rub. Bear in mind the Focus RS wheels weigh close to 27 lbs. Maybe suss can chime in on this.
16" wheel v.s. 19" RS wheel and recommended tire on Tire Rack for the 19" size with the RS wheel.
ST stock wheel size v.s. FRPP MK2 RS wheels with recommended tire size from tire rack.
People have not seemingly found the wheels to rub or to hit the steering rack at full turn under most conditions, not entirely sure the specific circumstances that brought about the rubbing that led to Ford deciding they needed to add the limiters, but the limiters give the car a worse turning radius.
If the 18x8 ET 55 wheel fits and it does, then the 19x8.5" wheel should also fit although it is pushed out a bit more towards the suspension.
I have the 18 TI wheels on a SE sport rack that came with 16's. I haven't had any rubbing issues. I'd be afraid to drive with a 235-35-19 tire though. I have enough problems with potholes and my 18s as it is.