I don't know, maybe this has already been brought up, but I've just recently started working on my last 3 battle jobs, namely, gla/mrd/lnc. I completed the gla 20 quest and decided to work on guild marks, and I have to say I was very disappointed in the xp per mob doing 20 leves @ Horizon. I set the difficulty on 2 stars, which is 3 levels above me, and without GA got 123 xp per mob ><.
I don't understand why this is so difficult for you to get this right. I fight mobs outside of leves +3 vs my level and I get decent xp. I have to say I'm not looking forward to accumulating GM's under this very unbalanced scenario, and I do hope you're planning on leveling the playing field, xp wise, for guild mobs. (Note: I'd appreciate no comments about the reward makes up for it, because without GA it doesn't, and sucks in parallel with the mob xp.)