I currently mainly raid 25 mans, but I was 10 man all of cataclysm, so I do have some experience. I personally believe that holy has its place in the raiding scene but disc is extremely powerful on 10man. Atonement dps is extremely beneficial due to the smart healing and the amount of dps you can do. Fights with damage increases (jin'rokh fluidity, horridons cracked shell, ji-kuns primal nutrient ) make disc atonement even better. I have seen up to 500k penance crits on Horridon, that is a lot of healing you can put out.
I have played holy and disc for heroic Iron Qon and I found disc to be better overall. Especially for 10 mans because you can cap spirit shell very easily. I would give it a try and see how you like it, the fights are absolutely doable as holy but disc will out perform holy on many of them.
I hope I didn't ramble to much for you, good luck