The reason for their effectiveness is that they can reach very high amounts of armor penetration (arpen). Most mdps classes can, at BIS, reach around 60% arpen via gear (including 4pc Triumphant proc), talis and renown (normal Crit, FS build).
On top of that, the tactics are added.
- MSH, Choppa, Slayer have a +120 STR + 120 WS -120 TOU tactic
- Mara and WL have a +25% armor pen tactic for their most important AOE skills
- SW has both a +120 BS +120 WS -120 TOU tactic and a +15% armor pen tactic on their Skirmish (AOE) tree
This means that instead of ~60%, the classes end up aproximately at:
- MSH (sacrificing all crit): 82% arpen
Let's now take the case of Mara for example, at 85% armor pen and lets pit him against a BIS tank and a BIS light armor healer.
The tank will have around 4900 (111.4%) armor and 4% reduced armor penetration (redarpen).
The cloth healer will have around 2900 (65.9%) armor and 5% redarpen.
Running the calcs we get that against the Mara they will end up having these physical mitigations:
- Tank:
Physical Mitigation = 111.4×(1-0.85+0.04) = 21.2%
Damage Taken = 78.8%
- Healer:
Physical Mitigation = 65.9×(1-0.85+0.05) = 13.2%
Damage Taken = 86.8%
As you can see, increasing the Physical Mitigation hardcap (currently at 75%) will do absolutely nothing. Even the tanks are just barely reaching 20% mitigation.
And just to clarify again; Armor has no caps. Only Physical Mitigation has a hardcap.
But what about Armor talis?
Lets examine your other suggestion; Armor talis: 136 --> 180
Currently only healers run those talis so lets check there. In the above cases I assumed the healer has 6 armor talis on, so lets add to their armor total this extra hypothetical armor:
Healer armor = 2900 + 6*(180-136) = 3164 (71.9%)
Redoing the calc we get:
- Healer:
Physical Mitigation = 71.9×(1-0.85+0.05) = 14.4%
Damage Taken = 85.6%
Compared to the previous 86.8% they would only be taking 1.4% less damage. Barely any difference.
As you can see, both proposed solutions would do basically nothing to reduce the amount of incoming physical damage.
The reason for this is that current armor pen% numbers are absurdly high.
To give you an example, to reach the physical mitigation cap (75%) without reduced armor pen vs the worst offender currently (WL at ~88% arpen) you'd have to have a whoping 27500 armor (625%).
There are two actual solutions here if the devs want to tone things down:
- Bandaid method:
Adding more reduced armor pen% into the game either by adjusting abilities/gear bonuses (DOK's new Covenant of Tenacity does that basically)
- Direct Method:
Reducing armor pen% availability by either nerfing gear sets, tactics, etc. Alternatively they can tune down the scaling from Weapon Skill (e.g. 18 WS --> 1% arpen, instead of the 14 WS that it is now). The later should probably also be acompanied with a scaling down of arpen% tactics or else thing change would risk favoring certain dps classes (Mara,WL, SW) more than the rest (Choppa, SL, MSH)