Here are my latest wild assed guesses:
OK, the actual FHI specs for the SPT/STi kit is:
38 N/mm F
33.4 N/mm R
(source: SOA, though they quoted N/m not N/mm).
This looks very close to the relationship between what I'd calced for front and stiffest possible for rear (lower coils bound).
That gives us for STi/SPT:
217 lbf/in F
190 lbf/in R
If you then take my prior comparisons between the springs -> 27% stiffer at front, and 107% stiffer at back (stiffest) and work backwards, then you get:
US WRX sedan:
169 lbf/in F
92 lbf/in R
Note how soft the rear is compared to the front, compare to either the rates Jamie Drummond measured for the Aussie WRX and the STI rates.