I found the stat weight to be .169 compared to 1 point of strength, Meaning 80 Tenacity is Equal to 13.52 Strength Bonus. While 80 DET is equal to 27.04 Weighting it based on ARR Nerfs at 0.338.
As for Damage Mitigation , I haven't a clue, Be we can assume that, i'd be set at half of the damage dealt.

So that means 80 Tenacity would be equal to 13.52 Str and 6.67 Armor Bonus giving it a stat weight of .253(5)
Honestly at that rate, I kinda see what they were doing, But it still forces the stat into gear where it Doesn't need to be, as Mitigation should be used as much as possible when you main tank that's for sure, but for off tank roles were it's not the primary concern, It's Not the best thing to take with you.
I think SE Forgot there is two tanks in the Party, but haven't recognized that party's aren't always designed around making mistakes.

Honestly even if it WERE overpowered, Only 1 tank in the group would take it over offensive Options, think about that really quickly.