Tenacity

[Deceptus]

At ~i500 differences between minimum and max Tenacity should be pretty noticeable.

A tomestone set has 2028 Tenacity at base (.951 damage taken multiplier) which can be increased to 2954 Tenacity (.922 damage taken, a 3.1% decrease in damage taken) with melds and food.
Mixing Tomestone gear with Edenchoir gear for minimum Tenacity gives 580 Tenacity (.994 multiplier)
Mixing Tomestone gear with Edenchoir gear for max Tenacity gives 2872 Tenacity at base (.925 damage taken multiplier) which can be increase to 3845Tenacity (.8975 damage taken, a 3.0% decrease in damage taken) with melds and foods.

The difference between minimum and maximum mitigation is pretty big this tier with a 9.8% difference in damage taken.

And that is essentially a permanent Reprisal debuff on the mobs.

[Ultimatecalibur]

Im not a bit wrong here, the 20% from the trait is still present in the damage formula and is interacting with tenacity always http://allaganstudies.akhmorning.com/guide/damage/, so it will suffer the same diminishing retuns as any other mitigation buff as they are all multiplied together.

We now exist in a perpetual scenario B as of ShB where the tank stance defence buff is always active, so is always interacting with any buffs you use, tenacity or otherwise

While the mitigation from tank mastery is calculated in game the problem is your frame of reference. Going off of how much damage non-tanks take is a very bad one as Tankbusters are all calculated based on the damage tanks would take, not how much the dps or healers would take from one.

Tankbuster base damage is likely calculated on the devs end by taking the expected hp of the tank and then multiplying the damage dealt by 1.25. This means a 50k tankbuster is actual a 62.5k attack but was designed to be a 50k attack.

Mitigation is applicable based on the following order:

1. Base damage - The actual potency of the Attack (requires a naked dps/healer taking the hit to see)
2. Tank Mastery - For all Tanks this is a .80 multiplier. The effective base potency taken by the Tanks of a certain ilevel.
3. Defense/Magic Defense - Mitigation gained from Gear ilevel. (Only way to actually see this mitigation is to take the attack naked and compare it to taking the attack in your gear)
4. Tenacity - First actual variable mitigation. At i500 this will be a multiplier between .994 and .8975.
5. Block/Parry - RNG procs that will apply a .80 multiplier to all non-Shadow damage (Block) or a .85 multiplier to physical damage (Parry). Block has priority over Parry so any increased mitigation from Block will diminish the mitigation gained from Parry. Paladin and Gunbreaker can influence the proc rate of these.
6. Cool down 1 - first cooldown used
7. ...
8. Cool down N - last cooldown used

Going from 1 to 8 might show the total mitigation achieved by the tanks in comparison to the dps and healers but that information contains extraneous information that is not useful to the comparison (1 through 3 will be the same for all tanks for the same ilevel which renders them moot). Using them is in effect an attempt to pad out the mitigation to make Tenacity's benefit look inferior to its actual in play benefit. 3 to 8 is where the relevant Player controllable mitigation is happening.

And that is essentially a permanent Reprisal debuff on the mobs.

Pretty much and it will likely only get better next tier with 5.4.

Much of the problem with Tenacity in StB was that the amounts required to get anything close to a relevant amount of mitigation was close to impossible.