Im not sure how the factory servos are powered on the focus rs but you will need a strong vacuum that you wont really get from a turbo engine so you might need a vacuum pump as well.

I personally cant see your need for a different peddle box in a rs, if it where me I would put the cash that your gonna speed on peddle box, master cylinders, reservoirs, servo, vacuum pump etc in to the disc and calliper set up.

The above is gonna set you back £1500 at least a big set of Ap's has gotta be money better spent.