This is quite simply factually incorrect. Disc are still very strong healers, and certainly still shine during low-damage phases (or on fights like Sha with no damage at all), but they simply aren't OP in the same fashion as they are in 25-man where they can fully benefit from uncapped DStar, etc - and in 10-mans, until they are wildly overgeared, their low "burst healing", for lack of a better word, is certainly noticeable when 2-healing particular fights.
Case in point: The currently ProRaiders rank 5 disc priest, Donfarfuzio, played for my guild up until last week. Despite being the rank 5 10-man disc priest, he was routinely beaten in terms of throughput by our shaman on any fight where there was more than 450ish k raid HPS to be done over the course of the fight. There is a certain, rather low, point where disc simply plateaus and cannot provide any more throughput, and this is a far more important deficiency in 10s than in 25s.