I have a 20 mm bar on a stock suspension, and I can oversteer on dirt like crazy - feels good that way.
I can hardly make it oversteer on pavement, but there was one time at night when I was surprised by a deer and braked quickly and swerved at the same time and the rear kicked out in an unexpected, though not insurmountable, way.
Can spin it like a top on ice, but I doubt that has anything to do with the bar.