I'm not so sure about your example of 8k vs. 50k. In a non-CLS, non-AP (armor penetration) fight, this shouldn't really happen. I had this a few times as well, (not really a monk/sorc matchup though) but it turned out that there was an explanation for that (unrefined gear, honing, PvP damage bonuses, guild skill. title, etc.).
The good thing about a single defense formula is that the same rules apply for everyone.
Crit. JT vs. frozen: 3584%
Crit. VS vs. frozen: 3200%
Crit. Flame Explosion: 3080%
Critical. Tempest: 2800%
Crit. Moonlight Dance: 2880% (can be doubled)
Crit. Shadow Explosion: 2140% (can be doubled)
Crit. Vulcan Arrow: 2612%
Crit. Sacrifice: 2530% (add. damage via HP)
Against knocked-down targets:
Guilotine Fist: 1560% * 2 = 3120%
Brutal Strike: 1300% * 1.3 * 2 = 3380%
Rage Strike w\o BT: 1140% * 1.3 * 2 = 2964%
Shield Cannon w\o BT: 1160% * 1.3 * 2 = 3016%
To sum it up: Similar starting value (physical skill effect) => similar potential damage.
That's why I first compare the physical skill effect and then test the damage agains low-level pre-AoV mobs first. They don't have a meaningful defense or damage reduction and PvP bonuses don't apply either, so it's a good starting point. Thereafter, I test it in a arranged duel. Here, you can check things like parry, dodge chances, as well as other things like aiming issues, speed, etc. But never make the mistake to use Colo as a reference! And as I've mentioned in a different thread already, I rarely if ever use random PvP encounters as an argument. Because then, you have to consider gear swap, unknown seedrune percentages and other things as well.
The main difference between tanks and non-tanks should be that the former focus on defense, whereas the others can focus on higher attack rates, higher damage or higher crit. chances. And this is partly the case. A BM f.e. would have to choose between Bear Form/Ymir or Grizzly Form, which then defines whether he plays a tank or DPS character. But there are some major issues:
The rates are too high. A 55% vs. 75% defense f.e. results in a 80% damage difference. That's also the reason why armor penetration is so dangerous. And of course, this creates a high discrepancy between normal attacks and DoTs // heals. That's why I always suggested to bring this down to an average of 20~30% again.
High damage compared to hitpoints:
Higher defense rates become irrelevent against non-AP attacks eventually (tanks can achieve 90%+ defense, but there's a cap at 75%), but damage knows no cap. And if an attack deals 200k damage against certain PvE mobs, how could this be balanced against other players? So the damage-to-hitpoint ratio has to be altered drastically.
High vigor // cast speed:
An example: 50% vigor causes a Brutal Strike to have a cooldown of 10sec. So even if that BM does nothing in between two Brutal Strikes, he would still have a higher DPS than with a constant Rage Smash. And how is a Jupitel Thunder with 0.4sec casting time and a < 2sec cooldown supposed to be balanced towards a Lightning Bolt?
Look at the damage of a Rage Strike without Battle Tactics. You see, even if you remove BT completely, a Warrior would still deal more damage than a Wizard, Soulmaker or Ranger. So how is this skill supposed to be balanced? And there are many other examples - at different scales of course.
Unbalanced stat bonuses:
Although it might sound cool, that classes get specific stat bonuses, the concept is highly flawed. Is the hit bonus of a Soulmaker as powerful as the ATK bonus of a Crecentia? And whereas a BM without Grizzly Form once had the same hitpoints & defense as a Rogue/Assassin or Ranger, that's not true anymore. But is a higher dodge or crit. a good compensation? So that's why I would like them to be removed and rather see a better skill balance instead.
So to sum it up:
Would a 'magical' armor solve the problem of a Battle Tactics? Would it solve the vigor issues I mentioned? Would it solve the overall high damage output of players or the extreme differences between 60% and 70% defense? - I don't think so. And that's why I said that the game doesn't really need it. Even games like Diablo, Torchlight, Titan Quest and others games struggled to find a 'balanced' concept. So let's address the apparent issues first and see how it works.
And Gravity as promised class balances over a year ago, but I doubt that this will ever happen (you could annoy p2w players who invested in broken knights) and I doubt they even have a proper concept (just look at the latest Rogue/Assassin fixes).