MK3 and MK4 power armors (bigger grid, larger inventory), also adds various equipment (Night vision MK2, Energy shield MK3, Fusion battery, Singularity...)
MK3 shield is meant to be an upgrade to the mk2 shield. However, this is not the case.
Currently 4x Mk3 shield is 1600+100hp/s, 9xMk2 is 1350+108hp/s, and they take same amount of space. This is not significant enough of a upgrade, and I suggest buffing Mk3 shield by 2x.
The regen is supposed to be a little lower, in exchange for the extra HP, but you're probably right that it's not really worth the upgrade. I'll probably increase each Mk3 shield by 100 each (so 2000 in the above example), which would make the Mk3 about 50% better than the Mk2, and barely worse on regen.
Thanks for the feedback! Every little bit helps make the mod a little better and balanced.