there's nothing wrong with your math. of course your estimate of demo shout is just that, a guess, but it is probably close enough for this discussion.
both imp demo and imp tc are amazing mitigation talents, however, there's no reason to spec for imp demo AND imp thunderclap. pick whichever one less of your guild's warriors have, and go with that. you only need 1 warrior in the raid to have each talent, and a spec which includes both somewhat limits your options as far as some of the goodies in the prot tree go.
we have half our tanks spec'd imp demo + piercing howl, and the other half for imp thunderclap. (actually it's not quite half, more went with thunderclap than demo, because it's incredible in 5 mans for multi-target tanking). we do have one tank with both talents, but in general having all of your warriors spec that way just seems silly.
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I've long thought that these two talents are overlooked, the former more than the latter
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this is where you're wrong. I'd venture that every "high end" raiding guild has had one person spec imp demo at least since BWL. The first time I recall imp demo being talked about in a big way was when people were discussing getting 1-shot by broodlord.
of course imp tc was changed in the 2.0 patch to be equal to (at level 60) thunderfury, so a lot of guilds at the time who had 1 or 2 thunderfurys didn't pay too much attention to the talent until the expansion came out, and thunderfury scaled down with level.
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A side thought is that if the 18% damage taken is acceptable for the encounter (non-improved) then the tank with improved talents is in a position to shed 100 defense or so for a comparable amount of stamina or threat (hit + crit) to enhance his performance.
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unless a tank is hitting 590 defense (not possible with current gear, or at least, not practical) taking off 100 defense will never be a good idea. all bosses should always have imp thunderclap and imp demo on them at all times, and I think we can be reasonably certain that blizzard has tuned them around these debuffs being active.
This stuff, it's not exactly news.