As for the mitigation - on all my 3 tanks I started to see the difference at around 51% (on T3 bosses). My DT and guardian have both ~55% and I don't feel much less durable than our maintanks - if only I knew how to play...
Heavens Wol Do is just the best tank polearm you can get nowadays without raiding. Yawg, I guess the epic one you posted was a former world drop?
I am tanking with S&B nowadays (aggro & survivability are just better ), but polearm is still totally viable (one of our T3,5 tanks is pole-spec, he got the pole from Zaal yesterday^^).
I think there is nothing wrong with going for Improved Defensive Stance, I took it back into my build as well. It is a little improvement, but it still pushes your survivability.
Volcanic Rage is a must (imo) when going polearm, since you don't have powerhouse.
Will I notice a big difference between 47% and 50%?
Im using reaching strike as a weapon.
If you have some spare time for solo then get Heaven's Wol Do - it will improve your mitigation and hate.
I noticed a difference between 49 and 50% - I was often on Impale duty on Gyas with my carnie. With the same party setup (mage shield, green heals) I had trouble surviving at 49% without blue heal from time to time, while at 50-51% I was barely loosing any health.
I don't think anybody has numbers to prove it, but it's just there. At least it's how it works on raids, dunno about 6-mans - with a good party you can tank in really crappy gear, it's easier to notice if your party sucks than your armor performance...
Ah, don't forget that a lot of trash is not to be tanked (f.e. the jang in Celestial Necropolis), but to be CC-locked. If you're getting pwned by them it's your party's fault
Just remembered something - there are a few bosses that crit non-stop (f.e. Olik) and then the critigation can come in handy. But high crit chance/amount can always be substituted by some good raid armor or an off-tank