Not an exhaustive test by any means...

But boosting seems to yield 10% vs groups of same level

Boosted w/ 110 and 4 lvl 24-26 (with full heirlooms and 300% RAF)....was 63 EXP on Trash in Ragefire (24 or so was RAF 300% bonus)
Bosses were 342
Final completion was 40,000ish (with 27,000ish being the RAF 300%) so 12-13,000 w/o RAF


Same group subbed out 110 and lowest I had was a 37 which allowed queue still

Trash was 642 exp...didn't try a boss yet as I'm not sure if healer can handle