Spamming arcane blast is not reasonable. Any good arcane mage will be cycling his spells to match his mana regeneration. For an arcane mage, more mana = more dps, quite simply. Arcane Blast spam is just a fun theory that doesn't happen anywhere except trash mobs (or maybe the next Huhuran, should there be one).

Adding a shadow priest to my group increases blast cycle dps by almost 100 points.

This means you're putting mana tide in a shadowpriest group rather than a non-shadowpriest group, i.e. giving tide to people with VT regen rather than those without access to VT. The value of mana regen likely diminishes as you get more*, so mightn't it be better to swap the shadowpriest into the shaman's group for tide? Or dropping Tide in the healer group instead and innervating the shadowpriest.

* e.g. a mage can scale up their DPS by consuming more MPS: wand -> scorch -> fireball -> arcane blast. The DPS gain you get by adding another 100mp5 is going to be less the higher up the scale you go, in the extreme case you can't use the mana fast enough and the DPS gain is zero.

(Edit: tangent but because Vontre mentioned it - an arcane mage will scale with mp5 the best of any spec, since they have the best arcane blast to use. Their DPS gains will still diminish as mp5 is added but they could still be one of the better targets for MTT when they have a spriest already - where a fire mage might not and an affliction warlock certainly wouldn't)

I'd have to disagree about the usefulness of mana tide to a shadow priest-the biggest benefit a shaman brings to a shadow priest is wrath of air, which alone brings me 15Mp5 in addition to far more DPS. In addition, mana spring helps a lot, but I don't really NEED mana tide during fights, and I'm sure those people I'm grouped with also feel that effect. Ideally, I would assume a shaman being switched in and out of groups to make the most effect of mana tide, but that takes a lot of coordination that my raid leader simply hasn't gotted used too when dealing with shamans(Having 1 raiding shaman in the guild kinda sucks. =/).

This is true, because full-stack arcane blast is very mana inefficient. With a shadow priest in my group I could sustain cycles with 2-3 full-stack blasts, but that doesn't mean it is the best use for the mana. Mages, like any dps caster, have a certain threshold of optimal mana regen where stacking more will have less effect, or possibly none (diminishing returns). A single shadow priest usually fits this bill rather nicely.

The advantage of arcane is that the scaling up with mana is almost indefinite, the mana will never go to waste on an arcane mage even if it's better spent elsewhere. However, arcane also scales best the other way around, providing the best efficiency for mana-constrained situations.

I get the most benefit from an Elemental Shaman in my group, with a Beast Mastery Hunter being a close second. The more DPS I do, the longer I can sustain myself, and Totem of Wrath from a Draenei Shaman will pretty much hit-cap me in my boss gear.

Usually the warlocks in my guild are always the ones topping damagemeters, even tho we have very good geared firemages and shadowpriests. So my "caster-group of death" would be:

Shadowpriest: Shadoweaving, Misery and regen (their also very good dps on their own)
Resto Shaman: 1% hit from Racial, and 100+ Spell Damage, ManaTide totem, and dont forget HEROISM
Warlock: Heavy Affliction with 3/3 Malediction for the best CoS.
Warlock: Heavy Affliction
Warlock: Heavy Affliction

imo, easily the best caster-dps group. feel free to flame or suggest improvements :P

Try including some other classes, for start. My understanding was between dark pact and life tap affliction warlocks don't have huge mana issues. If you have fire mages, they need a shadow priest for anything long. Ice mages get the most benefit from the shaman if they are willing to time heroism with the water elemental, and they also get more from wrath of air due to the water elemental "double-dipping". Furthermore, affliction benefits less from heroism due to heavy use of dots which are instant cast and don't gain any benefit.

Shadowpriest
Elemental Shaman (3% hit and 3% crit > mana tide, when you have a shadow priest)
Fire Mage/Destruction Lock - classes that benefit from everything spriest + elem shm can give.
Fire Mage/Destruction Lock
Fire Mage/Destruction Lock

Much better.

Yeah, sticking affliction warlocks in a shadow priest group is pointless - they can sustain themselves without any problem. Fire mages/destruction warlocks are the ones that need the sweet, sweet shadow priest love.