Anthony:
I think you are correct. I didn't pay duty on the 73CS I brought up in 2004 but they charged me the duty on the 76o2 I drove through in August. Customs dude said go ahead and appeal but pay up or you don't get in to the country.
It's on my list of things to do. Along with the 5spd conversion, brake upgrade, recaros, suspension upgrades ....
Doug