Well no, you are missing out half the healing by omitting the PW:S Absorb and you need to think about total healing, since disc get stronger the more atonement targets you have. In this situation when you use rapture, it will be when you know a large aoe phase will happen, so those shields will be consumed. Also if you shield 8 ppl and cast PW:R the maximum atonement's possible is 14 not 13.
So that being said those 15s become 8*600% sp + (6*150%+14*300%) for PW:R(only hits 6 targets)+penance AND if you take Shield Discipline you get 8% mana back so in 15s you get 9900% sp total healing,for 22.5% mana. Thats not even factoring in Artifact traits or indeed the healing from SW:P and PtW which will be also ticking, i'd say thats pretty fucking good!!!!!
Edit: Btw i'm no coefficient maths wiz so the above may not be 100% accurate, but you get the idea. Someone like Atonement would be able to give a more in depth answer.
Edit2: And just to reiterate the point of why you need to think about total healing for disc is penance itself, in the above scenario it does a total healing of 4200% sp in 1 GCD!!!!!!!!!!!!!!