I, for one, do not recommend that procedure for the very reason you suggest.
Sheetrock is cheap, and 5/8th" sheetrock is common as dirt. If I intended to install a tub/shower in that alcove and wanted to have sheetrock for my walls, I'd consider no other waterproofing method other than Schluter's Ditra, which has been approved for that application.
There may well be some argument from your code compliance inspector, but you should be able to get it approved.
Otherwise, I'd recommend installing a CBU instead of your sheetrock. Should be no problem getting the code people to accept that as a replacement for the Type X sheetrock. You can install a moisture barrier behind that or a waterproofing membrane on the inside.
In any case, I'd recommend you add framing to the wall to reduce the spacing to something 16" on center or less, even if you intend to stay with the 5/8th" sheetrock.
My opinion; worth price charged.