The FastFix Depth Stop doesn't work in the same way that some depth stops do. The driving tip is spring loaded, so rather than the bit essentially camming out of the head of the screw when the depth is reached, the bit itself just stops spinning. So the head shape of the bit is somewhat irrelevant.
Just to be sure though, I've just put some TX20 woodscrews in to pine through plasterboard without a problem.