THIS is part of the disconnect in determining the WB factor between the two carriers - NWA's International flying is a premium pay difference (on our relative scales) making it a more senior position.
I wouldn't be bragging too much about that "premium" pay. A 7 year NWA A330 F/O makes as much as a DAL 7 year 737 F/O.
On a related note, I hear the CRJ700 is considered premium flying at many regionals too.
At the end of the day it's how much you make that matters.
Contractual values play a part in SLIs. If you merged with Kalitta, would you consider their 747s "premium" flying and slot them in with your 747s?