Zsun, have you ever tried soloing with an Atonement spec? I imagine the damage output would be significantly reduced, but I wonder if the Atonement heals, and clever selection of talents like http://www.wowhead.com/talent#bfMsrRcbcoroZhb0b +2 might give you more survivability without having to change gear (shadow gear + Twisted Faith = yum).
Actually no I haven't tried out Atonement. But with the amount of damage Smite is doing right now, and assuming Atonement heals for half of that, It's approximately equals to around maybe 5k heals per second. This is certainly enough for some Naxx bosses, but for some of the other ones I have been farming such as Malygos and Onyxia and definitely not enough for Alysrazor (DPS-wise), which I started farming as well recently. Since this is definitely not enough heals on its own, which means I must use Flash Heals or PW:S to compensate. The problem when Flash Heals and PW:S becomes involved is mana, which I was afraid that the lack of mana regen mechanics such as Masochism may hinder my progress.
Also, being in disc spec means I lose out on cool stuff such as Dispersion, and when soloing I prefer that over Pain Supp and Power Barrier. But that's just my preference.