Both the healing and the absorb of SC are affected by Grace - Grace increases the healing and then the absorb effect is 50% of the heal, that was already increased.
Anyways SC is very niche. Basically you will use it in M+ dungeons with grievous affix to quickly top off the entire group near the end of a fight (that's what I'm told BTW, I usually run holy in dungeons and disc in raids)
Also check this most excellent guide here: http://focusedwill.com/disc-guide-7-2-5/