Hmmm, I tried this with my 77 Pally by the onslaught camp by Naxx. Mobs there are 71-72ish Pally was getting same XP in group as he would out of group. Other toons helping to kill were 4, lvl 74 Shamans. I also tried the Athene Instance tagging trick in UK.....with the same results. So, Im back to group grinding.