it heals a lot as shadow because the chances are they have more throughput stats. Likely more spell power with trinket procs also. Remember to factor in mastery from healing spec and it should come out slightly ahead as disc / holy.
It did a huge amount of healing when I used it on HC Will of the Emperor(25) this week, only problem was that since it deals damage as well as flies like 500 km forward I sometimes broke the CC. Now you can say that's my fault (which it is), but for now I'll stick with Cascade for most of the fights.