You do not get full xp for a wz that doesn't have enough people. You can not power grind to 50 in two days doing wzs, believe me i have done them for 12 hours straight before. While xp does scale with level you will need many more wzs to go from 20-21 than from 10-11.
edit: I am not in EGA yet so I can't say for sure the xp for incomplete wzs is as diminished as it was on the last day of beta, but at that time xp was severly diminished.