Here's an idea with the pvp problem with the armor buff for sin tanks... put it high enough in the Darkness tree that only tanks can really benefit from it and not pvp hybrid specs.
Take Hollow and switch it with tiers with mounting darkness.
Then have Hollow read as follows:
Increase your shield absorption by [2/4%]. In addition when Dark Charge is active increase armor rating by [10/20%], also when you activate Overcharge Saber while Dark Charge is active damage reduction is increase by [12.5/25%] for the duration.
This way we get that armor increase Sin/Shadow tank need so much to help lessen the damage we take from the spikiness. Now understandably the percent I put in for armor may need to lower or higher to help balance us out between Juggy and Powertech tanks