Here's the thing - you will damage the backer no matter what you do. The half moon carbide Fein is much better in theory than in practice - burned one up in no time removing tiles from a shower. In the end I found it much more expedient and successful to use a wide chisel like a brick set, a flat bar, or something similar to pop them off. Found out the backer wasn't correct in the process too - normal gypsum not green board.
You have to remember, just because you remove the grout doesn't mean the tile is simply going to fall off the backer (no matter what the backer is), you still have the mastic holding it. You're still going to have to pry it up and risk damaging the backer. And believe me, trying a Fein (or Bosch, or Porter cable) to cut the tiles free from the backer behind the tile will fall short in every aspect. Yup, tried that too and did more damage to the backer than just removing them.
You should expect to replace some backer and repair that wall. A well done, properly done job and the repaired backer will never be noticed.