International pilots normally fly more not less block hours then domestic pilots and are generally more efficient. The NRT operation at Delta is however very inefficient on a block hour basis countering that to a degree.
If you want to argue about block hours per pilot a much better case can be made based on the fleet at Delta. Managements decision to operate a fleet comprised of every orphaned and bastard child aircraft in the world builds into the system ineffiencies not found at SW. Still even adjusting for that SW is far more efficient.
In 2011 their average daily aircraft utilization was 10.5 hours a day. Delta was at 9.2 on narrow bodies. That alone should require SW to need 13 to 14 percent more pilots per airframe if all other factors were equal.