the main reasons for differnce in hp between the two, jdm and us spec is the ecu tuning (its more agressive,fuel maps and timing they were tuned to run on a higher octane gas)and the fact that the jdm does not have an egr valve system.(the egr system dilutes the Intake
charge and robs hp.)On some of the older celicas esp turbo models the system was built into the head itself,this lack of egr in the head accounted for some hp in the jdm heads. its funny,i have seen some heads with the egr blocked off as if it was there but just blocked and i have seen some jdm heads that the egr was never a part of the design in the head!these heads might have a better flow advantage over those that were casted with provision for the egr system)i never checked the 7th gen us and the jdm to see where and how the egr system were on both of them cause it never came up.the exhausts on some jdm vehicles are different as well
ps: with the egr system that uses the vacuam hose to open the valve,i usually take the hose off and block it when racing so the valve does not open(this is for other cars i have never done it with my 7th gen celica cause its my "mobile office" and not my race car.