Trying not to hijack mogs thread too much just giving experience here.

I've done about 60 5's games this season and its been mostly with junky gear while my opponents out gear me.
The shaman brings about 1.5-2k dps more per character from the buff. They pump out in arenas roughly 22-25k each in current gear and the mastery buff itself feels like it adds 5% more damage. On a good game the shaman will do 18-20k and a bad game 12k. This is how I mark their performance. Also to note the Shaman is ALWAYS, targeted first. So he is getting stun locked etc while the dk's go to town. This actually isn't terrible since the shaman mitigates more damage blowing active cd's than a dk does its a weird benefit.
So the dps loss the shaman has is made up for in the amount if gives the others. Additionally the shaman gives the healing Druid 10% more spell power on top of the mastery buff which adds about 2k spell power and 6% more effectiveness on my heals through mastery. Netting my regrowrth spam an additional 10-14k heal per cast while in pvp mode.
additionally a healing stream totem is dropped or healing wave.
Plastic but not least solar beam is and can be a game changing opener. 4 second targeted aoe silence really sets the tone.

Lsing the extra double grip makes the death grip rotation a little more spread out I did enjoy a never ending grip practically every 3 seconds. You can also perform an aoe stun via asphyxiate on 4 targets instead of 3. And doing two separate double remorseless winters really adds to the cc.

At this pint I'm back in the 1x Shaman boat and if I can further tune/hone the dps/burst if may be another reversal where I get 3 shaman and a dk. The opening burst power on the shaman is really what wins the game and right now I am not dropping targets as fast as I would like to be. They literally have to flop as soon as you hit them with the insane amount of "outs" every class in this game has for the,selves and others and I still need to learn all of the abilities. I sat there trying to burn down a warrior today while a monk literally made him immune to damage.

So lets have a class merge! Give me the dk's with the shaman 3 minute burst and we'll call it a day.