I personally still prefer using gems that give up to +150 HP each.
I'll use one of my characters as an example. My level 600~ ED has an original mitigation value of 1.16%. If I use 4 gems that give a 30% mitigation multiplier, I will have a total of 120%. Considering an additional 7.5% mitigation multiplier obtained through dedication perks, this value would increase to 127.5% (multiplier factor of 2.275x).
Application to Base Mitigation: 1.16% × 2.275 = +2.63%
In a 1,000 damage combo, my mitigation will absorb 26.3 of the raw damage.
In a 5,000 damage combo, my mitigation will absorb 131.5 of the raw damage.
So, even with the improvements, I consider the damage reduction low, and that's why I still prefer to use 4 gems that give up to +600 HP.