Hello. I have just boosted my 3 shammys with my 70 feral druid and part my 70 elemental shammy. And lvl 40 to 50 is just slow as hell.. Nothing much you can do about it.. I did ZF till 45, and then i power quested them from there.. Just take all the non gathering quests with your 70 to make it for fast as hell.. that is my best advice.. I did some BRD, baron, ST, etc.. but it seems that it was going way to slow as they start to deal so much dmg, and when you are pulling larger amounts of grps there will normaly be a few that runs away in fear and chain pull others and they normal runs right for you alts.. So go do some power questing after ZF, and then when you hit 50 start the Onyxia prequest chain which will eventualy send you a few times into BRD. Do not miss out that chain even if you might not want it done or finish it all up.. it is some insane xp for a lvl 50 as it was intend to be done at 58-60...