Yes, inertia is linear with mass and quadratic with radius but even so that still means BW's claimed halving of mass allows them to make the turbines 1.4x larger and have the same inertia. A 40% increase in turbine size at no inertial cost is pretty dramatic to me. But this is also assuming the material is distributed the same in the turbine design. If BW's wheels have more material at the edges than Garrett's then obviously some of that advantage is lost. Even so, a 10-20% increase in turbine size would still be a formidable benefit. Combine that with the new geometry and twin scroll housing and I don't think anyone has anything to match the 7163.
Where are these comparisons you've seen? I've only seen a handful of comparisons with the first gen stuff and next to nothing on the 7163.
Edit: Found a GTX vs GT comparison
Looks like the GTX isn't much better than the GT. The 7163 that Perrin tested would still beat it pretty handily below 3500 and be about equal above there.
And if anything, the GTX spools slightly later. That makes sense considering they have a more aggressive compressor but the same turbine powering it from the GT.