There will not be an informed response to this question.

No one has enough information to tell. Situationally, I'm confident it makes significant difference... but I'm also confident it makes none just as often.
We really don't know enough of the mobs attack or defense to find the ratio and how much the pet def gear will help. the pdt gear is much easier to figure out and much more effective.
I do remember stacking enough pet def gear on clot pets even from level 75~80 to get 0 damage from trolls occasionally. I could see the difference then.