I suggest reading up on KogboxKing's posts in the Shift-It forum. Here's a brief summary:
1.) The stock Focus diff is glass
2.) The PG puts extra stress on stock gears that are already over-stressed.
3.) If you plan to exceed 200 ft-lbs, you should upgrade to either a Quaife or Torsen ATB differential.
if you are planning on upgrading your tranny with an lsd.
My suggestion is to hit up KogBoxKing. He is one of the designers of the MTX so he knows just a lil bit about them.
I may not be cheap... but it will be the best upgrade you may ever do to your car.