I tried this out last night. The kill XP is reduced by roughly 80%. That is a fairly big hit, but stuff is going down much quicker. The mage and lock do a shadow/frost bolt at the same time (frost bolt has a slightly quicker cast time) and when the shadow bolt lands, the felguard charges and pretty much only needs one more hit from him to down the mobs...well, the ones I'm currently fighting anyway.

At lvl 71 or so, i would ungroup the lock as you can tag boost and get full xp on your mage for mob kills.
Does this work? I thought that when this was done the XP gain was based on the amount of damage that was done?