I have been running my 2012 Titanium in STF. The only change I made was a new set of wheels and Falken Azenis FK452 tires. Heck, it still has the original oil in it.
In my region my only competitor is in a Mazda2 but I have been able to keep ahead of him.
I think the new Focus can be very competitive in STF. Problem is there is such a small list of vehicles that fit in it that I don't see me having much to compete with. I could run in HS but that has had less competitors in my region.
We have a local class called Street Touring Open 4. Any mods, ST legal tires and 4 cylinder naturally aspirated. We also have STO6 and STO8. Forced Induction bumps you up a level, except for 8s. Rotarys run in 6. We had 6 people in it last event and I would have won that but not by much, so maybe I should move there.
Those classes are always the ones with the best turn out. A number of stock cars are always entered because many people would rather lose to someone that win against nobody. Seems to go against the Solo mindset of each car needing to be competitive in a class.