Tank Mitigation vs Tank DPS.

[Nedkel]

I have no idea where you're coming up with those DH numbers from, and you're entirely excluding crit for some reason. Tank BIS this tier have approx. 1300 DH, which is a 3.8% bonus, and around 3500 crit, which is a 14% bonus. Remember that damage stats stack multiplicatively on top of that. (http://theoryjerks.akhmorning.com/stats/crit/, http://theoryjerks.akhmorning.com/stats/dh/)

If you brought tenacity to absolutely maximum levels possible right now (3582), the damage and mitigation bonus would still only be 9.7%, not even close to the combined crit/DH bonus. At more realistic levels (around 2000), you can expect around 5% both ways like you said, almost three times less effective than crit alone. (http://theoryjerks.akhmorning.com/stats/ten/)

This isn't even beginning to go into other details, like the number of raid buffs that favor DH and crit (Battle Litany, Battle Voice, Chain Stratagem, etc). Overall, DH and crit are both much more effective than Tenacity. In short: everything you've said about the comparison in DPS increase is wrong.

Tenacity might be useful as a bleeding-edge prog stat currently, as I've heard of at least one E4S clear where a tank with 3000 tenacity saved a run with only 3k HP after mistiming a Shadow Wall, and even in that case it was due to a player mistake and not required. At absolute best, Tenacity is padding for progging where a tank might be expecting to mess something up. In any other case, using Tenacity is essentially preparing yourself for failure, because if it benefits you in any large amount it would ultimately be due to failed mechanics.

I am not advocating for making maximum tenacity and take the maximum possible tenacity from equipment, because for most of the players they will not have a choice, since ilvl is the first most important thing to focus on as a tank.
I am just comparing melds because here is where the choice we really have, tenacity with mix of D hit where you cant put more tenacity against pure D hit melds.

Right now the difference sit at exact 1.7% on damage and 4.3% mitigation, with few d-hit melds its basically about 1.6% and 4.0%.
My advice, melds whatever you want, it will still give you something useful, tenacity is not useless otherwise it would be buffed.