Under current game mechanics with the new "curve" system, it has created a diminishing return on the buildup of your tanks stats - AC and PRR. Unfortunately it's FAR too sharp a diminishing return. Giving up precious slots to fit in more AC may give a small return assuming you can get your AC into the monster's attack bonus, which is far from low in EE... luckily you only have to more than DOUBLE it to have effect survival AC (which is impossible... so that extra 50-100 AC you're gaining by turtling may earn you 10%). Going full turtle vs just using Mountain stance and a PRR item or two? 10% less damage. So here you find yourself giving up evasion and dodge where both cause you to take ZERO damage and face eat 450 damage while the clothy takes 500 if they get hit. Worth it, right?
With the fact that S&B is already a massive DPS sacrifice, the AC/PRR pass killed the turtle tank. 90% of your survival options come naked as anyone can get displacement clickies and either kiting at ranged or 25% shadow fade is much much much better than taking slightly less damage. We already face the issue of massive damage numbers in EE - unless you have a dedicated healer dumping mana on your head, you can't melee tank them anyways, and if you have that, you might as well have a tankish DPS do it. Who cares if he takes 50 more damage per hit if he's doing more than double the DPS?
For AC, increasing the multiplier shifts the AC into the range where it could be effective in EE. Unfortunately that won't stop it's diminishing return, so the gain of 150 to 200 is about half of that of 100 to 150, but it's still a sharper curve making that gain more worthwhile than it is now.
PRR is the bigger item on the table. There is currently no "glass cannon" vs "tank" - everyone gets gut punched for hundreds of damage, so not getting hit is the entire key. That makes dodge, incorps, concealment, etc a BILLION times more important than PRR. The first 50-70 PRR is of course important as that makes up 50-75% of the PRR survival bonus, but anyone can try to get that... but after that, giving up a single point of dodge, incorp, conceal is a massive survivability loss. The days may be gone of all or nothing AC tanking, but we still should have our tanks. Fully armored 200+ PRR turtles should be getting hit for a tiny fraction of what that robed Sorc is getting hit for - not 10% or 20% less... 80% less. The PRR formula really needs to lose its fractioner - a straight (1 – (0.99^PRR)). This would mean that the tank is soaking up 86% of the damage instead of 56%, but isn't that how it should be? Going tank would make a serious difference, and would justify hard hitting EEs. You really would have your glass cannons that take 500 damage getting 1-2 shotted where the tanks only get hit for 100... because, you know, they're tanks.
The PRR change along would bring the decisions between cloth to heavy armor to the forefront, and I know I'd go from 100% of my chars being light/cloth to a mix.