I replied to your other post, as long as the PB isn't skimmed then tile it as it is.. do not use ply , it holds less weight and is subject to thermal and humidity expansion issues..
could you not board over the plasterboards with 6mm hardibacker,saves ripping it out ? i get 6mm 1500x 900 hardibacker for £12.50 a sheet from TP.
and im with everyone else,ply is a no no
This existing plasterboard should be removed, if it aint skimmed then its surely been painted, strip it out replace with new, will take around a day to a competent novice.