Ok, firstly I don't know how difficult solo-ing and leveling as a full holy priest is nowadays, because I was never full holy while leveling. Nevertheless I am in the mood to rant on a bit (flame me if you will).
I leveled using what I eventually called my "b*stard spec". This was a 40/14/7 discipline spec. I took deep disc talents such as reflective shield and enlightenment over mental strength + power infusion, which I saw as unnecessary. Once I had these I went into spirit tap and improved pain, picking up the last 14 holy as an afterthought. Leveling was a blast. I loved healing instances like ST, and even BRD was fun despite its terrible size and layout. Reflective shield was amusing, spirit tap was great, and other discipline talents helped ensure my survival. All the way up to 70, I didn't consider myself seriously gimped in terms of survivability, killing speed or healing capacity.
Any pre-70 instance, healing is a function mainly of group ability and competance, and probably latency and spell selection, rather than spec. As long as your tank was competant enough to hold aggro, and had the mitigation of sword-and-board, he was healable.
I cannot see how solo-ing at holy one would have to drink every few pulls, even on double pulls. Setting up a spell routine using a minimum of damage spells and a good degree of wanding should see you at almost full mana by the time you take your next pull (ok, so it is spirit dependant). It is arguable whether burning a good deal of mana on nuking down a target with direct damage spells before drinking, or doing a couple then slowly wanding it to death before casting renew and moving is quicker, but as a stingy gold obsessed freak I know which is cheaper (without having a pet mage). If my previous and current killing speed was "slow", then "fast" must be incredibly quick.
Anyway, with your build; unbreakable will is useful in pvp and certain raiding situations, but in solo work wanding is such an easy source of dps. If you want to level from 60-70 (or do daily quests in a timely fashion, for that matter), wand spec makes a much more noticable difference. I "used to" think that silent resolve was a compulsory talent, until I analysed my deaths in pve situations and realised I was usually hit when a mob broke CC early or simply wasn't being tanked at all. It does make a difference, very occasionally I wish I put a few more points into it (just managing to pull aggro off thunder clap for instance), but improved power word: fort is probably a better choice. There are a couple of other things (1 point in martyrdom = lol for pve), but otherwise your spec has most of the important things.
Personally, I find myself consistently casting damage spells on the tank's primary target while saying "take some proper damage you tankadin freak so i'm no longer bored". This includes heroics. I dunno maybe I'm just lucky.