As far as I know the touring car guys went for ball diffs because of the reliability over a ball diff and the fact they couldn't put down the power as well with the stupid fast brushless motors they are using now. The newer delta winds put out massive amount of torque and revs their nuts off at something silly like 11000-12000RPM per Volt. That makes 66000-72000 RPM at the shaft at 6V
Back in the day of old geared diffs the major problem was the binding at speed and the fact that they channeled the most power to the wheel with the least traction. Ball diffs operate smoother under power. I suppose materials and tolerances have improved to the point where you can make a small gear diff wirk reliably.
Has anyone tried running the ATD diffs in Associated or Jconcept cars. It's the one with the ring in the middle of the diff gear made out of slipper pad material. Gives you a much stiffer ball diff. What would it do to the cars handling???