I think holy's biggest problem in tens is its mastery. They just fulfill virtually the same role as Druids in 10s. I can't imagine what they could change our mastery to.
It might seem clunky, but the only original thought I've had is that holy mastery instead of granting EoL instead builds up "charges" of healing energy on each person you heal, equivalent to a % of what you already get in total from EoL. Then a new ability to release that energy and heal the target instantly for whatever you've built up. The caveat would be the 15 second timer on the built up energy. It would give us something interesting, would go with the holy priest spec description of "damage reversal" and eliminate some of the forced overheating.
Or make it work just like a Prayer of Mending charge that has a duration of 15 seconds that is refreshed with a new heal and builds up until the target takes damage, then it is released on the target. Give the heal a stack limit of 40% of the Priest's health. This would be automatic, just like EoL, but like DA and Illuminated Healing we could actually start stacking Mastery before the pull or before damage (mana permitting) and have it instantly heal on damage.
What do you mean by competetive? Numbers? Usefullness? Take into account the dmg a Disc can, and will do etc.
Competitive means it will heal the same or more. A lot of it will depend on positioning and the way each raid chooses to do the encounter on 25man. The extra damage from disc on 10man may prove to be a big benefit early on.
I am going to go out on a limb here and say that to start with you might see disc being ahead because lets face it atonement is totally brainless, but as ppl learn the damage pattern you will see holy and other healers edging ahead.
I expect disc to do rather well on tortos as soon as ppl figure out the perfect timing for spirit shell and instants. If it doesn't then disc will probably be very weak in general unless the fight has an atonement buff.