Soundboard... is it worth the money?

Using this today... Customer wants 25mm. Batten fixed to party wall then 1 layer of standard pb with dB fixed over to top... My question is, won't the fixings from the batten help transfer sound through the wall? I suggested a stud wall Built in front (1 inch away from wall) with rockwall insulation then thr 2 sheets of board but he was worried he'd lose too much of the room.
I would have thought they would , why not foam with a resilient bar top and bottom