It will be an interesting year for AX in the HS Class.
The SCCA just announced that the SVT focus will also run in HS. That's good and bad.
Good in the sense that the SVT is no longer "outclassed" by the Mini Cooper S and bad in the sense that the SVT might...MIGHT be too much for HS.
You are not piloting an SVT. However, depending on the option packages your GF got from the factory, it might work to your advantage.
If the wheel package is a 17 inch rim, then your choice for Hoosiers is a good one.
I run 225/40R17. That is the largest tire you can run under the stock rule set.
They work VERY well on the focus platform.
As you are aware, the rule set is pretty strict for the stock classes. So, I would poke around the forum for opinions on exhaust and the like. I personally run the stock exhaust system and find that there is very little to be gained by going aftermarket. That's just my experience.
Brakes are another story.
Consensus on this site is that the best set up for AX is:
Centric BLANK rotors
Hawk HP+ pads
Race Blue Fluid (DOT 4 non-silicone based)
The Hoosiers combined with the brake set up above and you should be competitive in GS. You should be REALLY competitive in HS.
Indexed, you might actually beat your S2000.
Good luck and stay active on the forum. It is a great online community.