This is a crosspost, but I believe mods will agree with me that my original post is kind of in an awkward spot. This information will be good news to most people that play holy in 10 man.
Ran a sim under 10 man with sanc extended 33%. HPM / HPCT figures (adjusted by hand for echo, since simcraft does it funny):
CoH: 11.6 HPM, 40.4k HPCT
PoH: 13.7 HPM, 33.2k HPCT
Sanc: 17.8 HPM, 120.6k HPCT
After adding echo in manually:
CoH: 14.4 HPM, 50.3k HPCT
PoH: 17.0 HPM, 41.3k HPCT
Sanc: 22.1 HPM, 150.0k HPCT
I don't know if simcraft figures sanc's HPCT from a gcd, or a fractional gcd. Hopefully, a full GCD.
As you can see, simcraft is suggesting to me that sanc will be at least worth casting once in a while with the 4pc, even in 10man. Your sanc effic could be ~2 HPM lower or so, since I am mastery stacked. That is, sanc benefits from mastery but doesn't gain extra ticks from haste.
Times change. In 4.0 and 4.2 we knew that sanc had very limited usefulness in 10man. We only used it as a precast for spiking. In 4.3 it might be worth using that gcd to refresh sanc DURING aoe, a first.
Edit: as an afterthought, if precasting take the time to swap into IW, to make it even more appealing.