This is what I do: http://plusheal.com/viewtopic.php?f=...mental+agility
Basically it focuses more on renew as a filler rather than flash heal, and it grabs more mana regen for instant-casts (renew, pom, coh). You will also get a lot of SoL procs if you have a decent crit rating, which you should since your primary spec is discipline. I haven't had much of a problem doing this, though I play disc more than holy. My flash heals are 1.3 secs as holy instead of 1.2, so not a huge difference. Remember to work in Serendipity for your PoH's and you'll be fine.
I enjoy playing holy, but I have found that mostly the only time I "need" to switch to it are when there is another disc priest in a 10man (or more than 2 in a 25man), or for Dreamwalker and I actually get to heal her for a change. That's about it