I feel a bit silly - can't believe I didn't realize this earlier. I saw the signs, but didn't put 2+2 together.

"I have Big Bad Voodoo Raindance on round robin for 80 seconds of total mana regen."

Well... BBV Rain dance doesn't effect other party members!!! At least the mana regen part - just the caster.

So I switched to BBV - Slam dance which gives ALL party members in the area 30% more damage, and 20% attack speed increase.

It turns out a free 30% damage increase to all party members, for 80 seconds, makes a massive difference on the elite packs.