The stamina currently is lacking in power due to lesser damage overall(which we compensate with relequen procs which is broken now anyways resulting in half damage cut) and a need of building group setups specifically for stamina. What I mean is that for magicka you need not use very specific sets to their boost the performance, while for stamina it is a NEED to become competitive. The biggest problem is that you need lots of penetration to reach the cap, cause as of now both alkosh and tremorscale are needed and yet even then it does not guarantee penetration cap and the dps difference between mag and stam dds is still steep in disfavor for the latter. Also that piercing change is laughable. It doesn't really affect magickas but even further increases the gap. Bravo!

Not so long ago there happened a change of a nerf nature to critical chance and critical damage of all sets that possessed them and CP1.0. It was said to reduce the overall numbers of those so that other stats wouldn't fall too behind, to give a wider choice for players and to force a general loss of power. And yet here you introduce a 1piece set that gives an incredible boost to both crit chance and crit damage. Isn't it a bit, I don't know, contrary to your former intentions?
Also that mythic is inherently of medium weight implying it would be used to give staminas that extra edge to keep up with magickas. And yet it still works just as good for the latter.

Having those problems stated I'd suggest to revamp this set, or at least change one of its components. Instead of having, say, extra critical damage per stage, why not give it some penetration? This way it won't be forced on the magicka setups as much but will still help a whole lot for all the staminas while keeping them on a somewhat competitive level.