I disagree entirely. The 4pc T12 bonus awards something like a 5-8% heals increase on any given fights, whereas the 4pc T13 only gives 2% if you're lucky. The stat differences between 391 and 397 are too marginal to drop that 5-8%. The only time you should switch away from 4pc heroic T12 is when you can have 2x397 and 2x410 t13.
OP is asking about for shadow... as stated in the title of the thread.
In kilee,s guide he has all of the values for set bonus's, it's not worth getting the 2pc t13 if the gear you are replacing is Worth less pp. but if the gear you are upgrading to is worth more pp, it will be worth it. It's pretty simple math ^^ V