Jeff,
Once again, I appreciate the thoroughness of your insights. And much like yourself, I agree that building a 100% autox car in ST* will make it far too uncomfortable for street use. (By definition, SCCA claims that ST* cars are supposed to be "street friendly".) Even with Street / Stock class, an all out full build is not something you'd want to drive on the street everyday.
For better or for worse, I think your post brings up an inherent issue with the SCCA classing system -- in many classes, there are 1 or 2 ringer cars (models) that just dominates the class, and leaves every other model in the dust:
BS is basically an S2k vs C5 Corvette showdown
DS is a WRX spec class
ES is MR2 Spyder vs NB Miata
GS has the making of turning into a FoST spec class as well
HS is turning into a FiST fest; before the FiST came in, it was the Spec Mini class
STC (and ST before it got renamed) is Spec EF Civic class, and the darn car has been out of production for almost 25 years!
STS is the classic NA Miata vs CRX battle
STX is almost all FRS / BRZ now
STR is all S2k now
If you look at the list, I think only CS, FS, STU have a variety of different winning cars. Given the way cars are classed, I am really not sure if we can achieve parity between different models even when we are only considering a single class. Furthermore, I would personally also include STF into the list of having multiple different cars that are competitive -- but it is obvious that the K-engine cars enjoy at least some advantage over their peers in STF.
On an unrelated note, prior to picking up the 2, I have actually been semi-actively looking for both a RSX-S and an EP3 for quite some time. It didn't really have anything to do with STF or autox -- I just liked both cars a lot, and that K engine is simply amazing. Unfortunately, I am just too picky a person when it comes to used cars, and the specific models that I wanted -- the 05-06 RSX-S with the higher output K20Z1 and 04 EP3 -- are too rare, so my dreams with the K-engine never materialized.