It scales inversely from 10 to 25 man since you nearly always break the 6+ rule and therefore heal each target for a more insignificant amount.
And becomes more mana efficient by reducing the chance it will overheal....
Seriously, put down 2x Sanc, 2x healing rain and 2x efflorence in a 25m raid and you can pretty much go /dance while all the stationairy heals get the raid back up to full hp. More to the point, these effects do all that WHILE you are still able to cast all your direct heals. As Ultima stated, this is something you can cast to boost your healing pre-emptively while you're still casting your normal heals.
However, it's also the first thing you cut from your healing arsenal if you have any manaproblems whatsoever. (assuming you weren't spamming flash heal...)
Now stop bashing on our pretty faerie lights. The spell effect itself is reason enough to cast it on cd!