that you definately install the backerboard, but then I am concerned about all the layers of wall board attached to the studs. Sounds like it will be pretty heavy.
The shower and stuff, well you can get extensions for that.
Now, if your budget allows or if you are energetic enough, I would start anew with cementious backerboard and do it right up front.
If you add this to your existing walls, you have tub wall to wall fitting problems, pipe extension problems, weight problems, etc.
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