Y'know, the more i think about Spirit Link the more broken it sounds. Think about it: it's effectively a permanent 50% damage reduction on the MT. That's HUGE. Yes, it'll still be redirected, but the soakers will only be taking 25% damage, which will be easy to heal through. Yes it'll break after some damage (and i definitely agree that a) there will be more ranks of the spell and b) it'll scale with spellpower)) but do you think any healer worth his salt is going to let that thing drop? Even if they put a cooldown on it, I can see raids stacking restos again just to make sure the tank always has spirit link up (unless they put a forbearance type effect on it, which would make it effectively useless).
As a quick example, let's take a a random SK Gaming WWS of brutallus*:
Wow Web Stats
the MT took an average of 3130 damage per second. If he'd been Spirit Linked, that drops to 1665 DPS. The two soakers only take 832 DPS each, which a good resto druid could easily heal through using rolling lifeblooms and the occasional rejuv.
*Yes i know this analysis oversimplifies things - it's based off of a close to perfectly geared tank, it assumes damage is constant and not bursted, etc. Still, it's useful as a thought exercise.
I simply can't think of a way where they can balance between a) a guild stacking resto shamans and completely cheesing encounters or b) making having a resto shaman completely necessary in order to even think about beating content.