Yes, due to how XP gains scale you want to fight enemies as close to your level to get the most XP with sever drops in XP gain if the difference is 5 or more levels. If you are below the enemies level by 5 or more damage drops to miniscule amounts and XP from any kills being far less than killing the same tier of enemies even 1 level lower. If you are 5 or more levels above the target's level you will only get about 10% or less XP than someone the same level.
Sub-class XP is actually based on main class XP. Sub-class XP is 1/2 half of main-class XP up to level cap -5 (with the current level cap of 20 sub-class level cap is 15).
At the current time (without the UQs which have level 17s) only level 15 enemies are available and level 20 is +5 levels over them so XP from Rank 2 combat zones and Vet bosses is drastically reduced.