I’m not a numbers tuning gal, so I won’t hazard a wild guess at this. What I will say, is that more passive defense alone absolutely will not solve DPS shamans’ survivability problems.
This is a great write up on the problem that is the massive disparity in defensive power between PvE classes in the modern game:
One of the important points here is that passive defenses have a larger impact on raid survivability, where there is much more emphasis on constant AoE damage to keep the multiple healers busy, while mitigation cooldowns have heightened value in M+ where the biggest concern is being instagibbed by unavoidable damage, which can often come in the form of single-targeted effects.
Importantly, DPS shaman are woefully lacking in both categories, as most of our survivability takes the form of self-healing, and you can’t heal yourself through a one-shot.