Do enemy level matter on increasing "weapon efficiency"?

Im at max level 69 and finished most playthrough 2.5 except caramerax and the underdome.

I just realize that DLC on playthrough 1 i havent touched yet cuz I went immediately to 2.5

Does the XP im getting on weapon proficiency gets lowered since Im fighting lower level enemies if I do the DLC on playthrough 1?