follow being removed is a bummer which will hurt PvP quite a bit. With world pvp though the coordinated burst potential is still there and at this stage taking on even odds or even greater odds can be winnable outcomes. 5x Ele Shaman using their burst macro will definitely drop the first target and depending on gear can drop a second target through chain lightning. This being all instant and not much you can do unless you know it's coming.
Before I had 4 Shamans, I did 3 mages and they were also very effective in PvP and PvE but you lacked the sustainability with healing.
from a PvE perspective since the dungeons cap out at Heroic a lot of compositions should be viable. Even 5 Ele shamans should be able to power through all of the content.