I don't think there would be any problem importing your 8.1.7 export into the 8.0.5. database, as long as you haven't introduced any new 8.1.7 features while massaging the 8.0.5 database on your NT machine (those SQL*loader scripts)

I have done 8.1.5 to 7.32 and 7.3.2 to 8.0 and 8.1.5 without any problem. My databases were on NT but I don't think there would be any problem doing it between Unix and NT (make sure you use Binary mode when transferring the export file to/from the Unix machine)

- Rajeev