Nocturnal + SCH certainly works better in some small ways than SCH + SCH, but given that you effectively lock yourself out of two abilities (or lock the SCH out of his equivalents) since they don't stack with one another, why *wouldn't* you go Diurnal, as it gives you a more well-rounded healing kit for the encounter? Yes, Nocturnal has 5% greater potency, but Diurnal grants more than that in HoT ticks, and also grants 5% casting speed (useful for both healing and DPS--which isn't the case for Nocturnal's boost).

I know you want people to test it, but there's not really a compelling reason *to* test it. It's a less versatile healing composition just by its very nature.