I believe I need to put 5/8" drywall there in any case to meet the code here, no matter what kind of insulation I use.
For the XPS I meant two layers against the bottom of the joists and then the drywall below it - it just seemed a lot easier than cutting all those strips and foaming. There's probably good reasons not to do it.
You would need a 5" drywall nails to put that 5/8 drywall up + there is no 5" mud rings for the ceiling boxes ...etc
XPS foam sheets come pre scored 16" O.C...if the ceiling its framed right there is not much cutting to be done