given the weight and size of the area to be tiled i'd be inclined to replace with fresh plasterboard. you don't necessarily have to remove the existing plasterboard, you could locate the joists and overboard. just make sure that you use drywall screws of sufficient length to hit the joists.
at least this way you'll know your're covered.
be mindful that there may be pipes and cables running thru the joists.
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