I main shadow and I've pretty much chose the 2 set for T13 even if they are LFR pieces just for the fact I can safely execute without putting stress on healers when I crit death x2 under 25%. It basically allows me to use it on all fights, not just select, so you may or may not want to switch them out accordingly.
depends on what itemlevel the stuff is. the t12 4p is great, normally does more than 5% of total heal. I wouldnt replace that with a slightly higher itemlvl. (2p is nice regen aswell)
if your comparing t12 NonHc vs t13 LFR, stick with t12, same with t12 HC with t13 NonHc...
until you get 4p t13 at least. (rather for the itemlevel than the bonus, don't like them that much)
but as soon as its a whole tier upgrade (378 vs 397, 391 vs 410) take it!
just sayin, thats what I'm gonna do, no mathematical proof incomming.