Certainly I have noticed if I am rushing a low level character (IE 20ish) with a character of a much higher level (EG 75-90), the low level char gets very little experience. They get much more experience if I rush with a character in the mid-40's.
If you are talking about mixing <80 with 80+ in a Baal run, then no I do not believe there is any difference.
AFAIK you get one share of the XP per level, i.e. a high-level character will get more XP if he travels with a low-level character instead of a high-level one. However, low-level characters will often not have an impact on the killing ratio, so woith respect to the XP per second retio it would be better to travel alone or with a character who is as good at killing monsters as you.
The XP reduction starts at L75 IIRC, but that does only affect the share of the character and not the others.