I chose the two songs and rehearsed them on my church's Nautilus AT. The Prelude would use a string sample while the Postlude would use a strong consonant sounding church pipe organ sample.
There was only one rehearsal for the service (three hours before it began) so I didn't have any time to dig into the M8x's capabilities. I found a suitable string sample but was taken aback by the lack of strong consonant sounding pipe organs. The one sample I found on the M8x sounds just like the one Yamaha provides on the PSR-EW425 (I own one to practice on) which sounds a bit on the harsh side.
I settled on a substitute sample for the service and everything went well.
Having heard the school's M8x in action for other events (and it sounded amazing in those instances), I'm surprised that it seems lacking when looking for more traditional church pipe organ sounds.