Tonight (or next time I log), I will finally be getting my last piece of T8 gear, the chest. For disc, it will allow me to move that fifth piece around. The 4 piece bonus is impossible to ignore for disc, but for holy, I see a bit of leeway as far as gear selection goes.
In my holy kit, I had been mixing in a couple of T7 pieces for the extra PoM jump on certain fights. I am not sure how much it comes into play though. I think I am recasting it before it ever gets to the sixth person. It worked well for Sapph and a couple other Naxx bosses, but I can only think of two fights in Ulduar (XT and to a lesser extent, Hodir) where it would ever have a chance to benefit from the T7 two piece bonus. Are there any other places it may come into play?
Should I even worry about the T7 bonus? I don't gimp my stats too much by using two pieces, but the real question will come into play when the T9 two piece bonus (+20% PoM heal) happens. Probably pretty situational, but is it still worth considering?