You have mostly 397 and can make the 4 piece T 13, so I think now is a good time to break it. I think that the gained Intellect and Spirit will partially, if not entirely, make up for the lost bonus. Plus it makes your HR spam that much better.
If you are able to get 2pc 391 heroic t12, you will keep it the whole tier, 2pc t12/2pc t13 has better healing throughput than 4pc t13. And yes that includes heroic content. I have 391 legs and shoulders 410 the rest, and I get better mana regen and more healing with 2pc391/2pc410.