The document you linked to is out of date. It hasn't been updated since 2007.
High-flow cats are now legal for ST/STS. As was stated earlier, it must be installed in the stock location.
What you do to your Focus to be competitive in ST depends on how serious you want to be and will it still be your daily driver?
Since I've already been down this road with two SVTF's, I'll tell you what I would do if I was starting out from scratch trying to make a daily driver SVTF more competitive in ST.
Contact Auto-X-ZTS and get the rear sway bar he sells. Single best mod I've ever seen for an SVTF suspension. That's it for suspension.
Wheels and Tires:
Depending on your budget, stock wheels or 17x7.5 Rotas (Or something like a Rota) and a set of Dunlop Star Specs. Stock size is just fine. Getting a dedicated set of wheels and tires is preferable as any good ST class tire will wear very quickly with daily driving. ST class tires are also not the best in the wet. If you need a tire that can pull double duty, get a set of Sumitomo HTR Z III. The side walls are a little soft (Use higher pressure to compensate), but the level of grip is quite good and they last. They're also only $83 from Tire Rack.
CFM or RT high flow cat and a tune from Focus Power or Focus Sport.
That's it. Before you consider putting any more money into the car, spend that money on entry fees. Get out there and practice. You'll get the most improvement from good ole seat time.