Based on a (fairly accurate) model of what I cast, spirit gemming also wins at total output for discipline when comparing 1 int to 2 spirit. If you want sustained throughput in a mana limited environment (anything other than short fights or fights which grant you infinite resources), 320 spirit will give you more total healing than 160 intellect. My first paragraph (where I said I have been gemmed fully intellect) might seem a bit strange after saying that. The reasoning is this: on some (many?) fights the damage patterns are such that you can spend the majority of the fight getting rapture, keeping pom up, and spamming atonement - you only switch into "maximum throughput" mode at a couple points during the encounter. Once you have enough mana to sustain max throughput during those periods and a fairly efficient atonement rotation outside of it, you don't really need more regen, but you can do more healing (and extend the window of what atonement spam will cover) by gemming for throughput. There have been a few fights where enrage was so tight that what I actually wanted out of the intellect was damage (our first Gara'jal heroic kill was right at enrage; he died as he oneshot the raid and everyone was dead, including him).
I still get slightly more out of an int flask compared to a spirit flask (converting extra spirit into more PoH's isn't quite as efficient as it first appears because, well, PoH sucks - additional PoH casts tend to overheal a lot compared to our other spells) but again it's quite close. From a more practical standpoint, with spirit gems and actually pressing my regen buttons on CD, I cannot really run out of mana ever so more spirit would be kind of silly. I do have a bunch of 160 int gems and 320 spirit gems I swap out for fights where I don't need as much spirit (alt is a JC so I buy greens and spam cut them for perfects; it's pretty cheap).
As an addendum:
I raided last night with full intellect gems including burning meta (although, this isn't entirely out of the ordinary for me as I mentioned earlier). I still flasked and ate int food. If our logs were public, I would have the #6 overall rank on Council (the only really relevant fight healingwise) with 108k hps - I've done heroic council several times now with different levels of spirit (the lowest being just over 8k spirit, but that was when PoM crits were bugged so all you had to do in order to do infinity healing was press PoM on cd and let it full heal people and leave a 200k DA) always ending up just over 100k hps: due to the zero sum nature of healing that's how much is needed to keep us alive. I would not try and argue that intellect gemming is easier, but if you are good at managing your regen cds it is definitely possible in specific situations.
For most discipline priests though, it isn't worth it.