Yes, generally heavy discipline is preferrable over heavy holy, at least for me. Inner Focus is key because it allows one large heal without interrupting mana regen. In most fights the mana saved from inner focus is enough to complensate from the mana saved from the bonuses to greater heal in the holy tree.
As has been mentioned, generally piecing together blues is better than your class specific set. You need to have around 3000 HP to keep from getting one shotted and a reasonable mana pool (~5000-6000) then focus on spirit and + healing. With a lot of +healing gear you can drop down a few ranks of flash heal or greater heal to be more mana efficient. Lower ranks get the same bonus from + healing (which is 43% of the bonus listed on gear for flash heal), so they gain efficiency quicker than the highest rank. You will do lower healing rate, but healing rate generally isn't a huge issue as you won't be the only person healing your tank in most cases.
The points in holy are somewhat debatable. Generally if a healer gets aggro it's because something went very wrong. So subtlety is even a questionable place to put points. My experience is that a healer gets aggro in one of three cases:
1) The MT dies, in which your guild should have a contingency.
- In this case every healer has tons of aggro built up anyway, but the boss will likely go after damage dealers first.
2) A banish or sheep pops and the assigned warlock/mage is too slow in re-applications
- This is unavoidable from the standpoint of the priest... subtlety won't help this one
3) The boss has a special ability that punts the warrior out of aggro range, or wipes aggro or something similar
- These cases are part of the encounter and are to be planned around. For example, Majordomo can teleport the tank and this wipes his entire aggro table. Subtlety won't help here, you need people to bring domo back to the tank... Requires planning around.
I actually feel that inspiration and holy spec are decent raid level skills despite that they are not the most popular picks. With priests having inspiration, and the amount of heals the MT receives, it procs on a regular basis. For example, if the tank is receiving 10 heals per 15 seconds from priests with a 13% crit rate, he will have +25% armor 75% of the time. I know one guild that has an overabundance of healers that designates one priest to chain cast Rank 1 flash heal for the inspiration proc and a shaman casts a low rank lesser healing wave for the similar armor buff that they get. These points are wasted on those that do pure fire damage however, as armor is pretty useless against those bosses.
If your guild is not organized enough to plan for some priests having inspiration, I wouldn't spec that way, but go with a more standard healing spec.
Obviously spiritual healing is a must. Beyond that, there is quite a bit of debate as to what's best.