Last night my friend and I were xping. Hes 73, I was 53 (20 level difference). I dinged last night and we logged almost immeditely after because of the amount of time we were playing.
We just now came back to the spot today and realized the XP we're getting has increased around 10-20% for the both of us. It's strange because I just leveled, but the XP went up. Question is, is there a penalty applied to groups if the level range of the highest & lowest players greater-than, or equal to 20? Or did the mobs get a slight boost in the amount of XP they give in the Imperial Tomb last night?
I have no proof for you, merely anecdotes. But i've often tried to powerlevel much lower level friends. For especially low level ones mathematically i should get more XP than solo but i don't, i get less. so as they level up and get closer to my level then yes the XP does seem to increase. I suspect that there's some cap on how large of a percentage a given party member can take and the excess XP is lost.