Great question Lars, unfortunately one that no one can really answer yet.
Of course questions like this are always extremely subjective- if you *only* care about which is quicker around Nurburgring, I think it would be very very close between the 22B and the USDM STi, since both have high hp/weight ratios and both have adjustable diffs.
UK's EVO magazine (NO relation to the car!) had a killer test in it's Aug 2001 issue with the P1 vs the RB5 vs the UK 300 (all Prodrive specials), vs "grey import" JDM STi (note this is well before the new "twin scroll" JDM STi).
The results, again quite subjective, were interesting. The P1 came out on top, although the JDM STi was faster. They didn't care for the compromises the STi made for top-end power, it was very "laggy". They definitely thought both GDB models (the JDM STi and the UK300) were heavier and slower thru the twisties. They liked the RB5 better for day-to-day driving than the manic P1, as it's suspension was more compliant and it felt more nimble and more forgiving on the kidneys.
It's the same argument as EVO vs STi- in nearly every faceoff the EVO wins on the track, but for actual everyday driving, i.e. a car you could actually live with, the STi usually wins. 8)
That super-harsh competition style suspension kicks arse on the track, but sucks for real-life driving. This is the greatest criticism against the fabulous 22B as well- it offers great Subaru styling and EVO driving. Whether that's good or bad depends on what you truly intend to do with the car.
Of course, this all means nothing until the USDM versions of both the EVO and the STi are really put thru the ringer in tests- and compared to each other. Make no mistake, even without all the electronic gizmos, the USDM EVO is going to be one helluva car. AND with a slightly softer suspension, for us soft Americans, it may very make the EVO much more liveable, as well. Do not make the mistake of dismissing them lightly...
Which (finally) brings us to the USDM STi.. I think it will be awesome! I believe the suspension will be a tad softer than JDM versions (I hope so actually), and the biggest complaint, the turbo lag, should be much alleviated with the extra 0.5 liter. If you want it stiffer, hey, that's easy to fix... and keep in mind that in some situations softer can be faster- just not on the track!
My $5 worth,