Okay, I'm a bit torn here. I play a shaman myself, but I raid as Elemental, however my partner in crime is raiding as Enhancement, and now with the patch coming live its time to decide.

a) We always have BoS up in raids. Neither of us can raid without it these days
b) Threat is a major concern for both of us.

On my behalf, its easy. [Enchant Cloak - Subtlety] wins because there is no viable alternative.

However, for enhancement, we are torn on the "best". Subtlety would result in a very minor threat reduction for melee, over 200k damage it would result in a minor change of ~3k threat. ( 0.7*0.98*200, vs. 0.7*200 )

This is such a minor change in threat that its negligible, and I'd be led to believe that [Enchant Cloak - Greater Agility] would be better?

Otherwise, if someone would please tell me that I'm wrong and threat modifiers on gear is additive, I'd be in love and cheering and can finally lean back and say "there's nothing better than Subtlety around". However, I believe that my world of fluffy clouds and carebears will self implode on itself if I ever start to believe that.

Really it all comes down to what you mean by this.

If you are currently threat capped in most encounters, then you will want the subtlety. You are already dealing the maximum damage you can, adding anything to increase damage would only cause you to throttle back more. A net result of the same amount of damage.

On the other hand, if you still have a decent margin between your threat levels and that of your main tank(s), then agility would definitely be beneficial.

On a matter of personal preference, I would go with subtlety. Any passive threat reduction is welcomed. My damage is far too spiky already.

Is this really worthy of a thread devoted to it?

No.

If only there was a huge dedicated thread to everything about Enhancement Shaman already.