I have been thinking about this for a while. Well, ever since STF was announced. I think my choices would be:
1: 2008 Focus S coupe
2: 2005 D23 ZX3 (2005 because it's easier to get decent off-the-shelf suspension easier than the 06+)
3: Early ZX3 (even lighter than the coupe, but not by enough to overcome the disadvantages IMO)
4: SVT Focus (I don't know any MY differences, so whichever low option model you can find)
Why the S Coupe?
The S coupe was dropped after 2008, the SE and SES models had options, which add weight. I'm guessing the steering wheel buttons, spoiler and the SES fogs are probably more than 30lbs, and the sync system is probably heavier than no sync.
The coupes were the one of the lightest Focii. 2008 coupe (2588lbs), 2005 D20 ZX3 2654lbs (D23s are heavier), SVT Focus 2750lbs. And I'm guessing that the coupes have more to lose in STF trim too. I'm betting those airbag seats are heavy.
They also have aluminum brake calipers for slightly less unsprung weight.
And they have the stiffest chassis; Ford added a dash bar.
So, 100+lbs lighter, stiffer chassis, lighter brakes and the same 3.82 gears as the 2.3 ZX3s. I think that outweighs a 9hp and 18lb*ft deficit.
Would a fully prepped Focus be enough to beat Andy Hollis in his 2100lb (prepped weight) Mazda 2 of doom? I think on a bumpy-ish, open, not-slalom-heavy course, you could have a chance.
Disclaimer I'm neither and autocross expert nor a Focus guru, so if any of this information is inaccurate, that sucks.