[wdfifteen]

I would check the area with a straight edge/four foot level to see how flat it is. Drywall can be significantly wavy. If it isn't flat I would rip out the drywall and start over with hardiboard. The drywall isn't an ideal backing and it doesn't make much sense to spend time bailing out a non-ideal surface. My experience with tiling has mostly been trying to take shortcuts and regretting it later.