It would take a ditch or embankment I would think. I suppose if you had a stock xB at stock ride height and with stock wheels and tires you could get it to flop on its side in an ultra high speed manuver if you really tried, but I doubt you'd get an actual roll. But you'd really have to try. Remember, you have Vehicle Stability Control (VSC) on your side helping to keep the vehicle flat on the ground as much as possible.
As mine sits, I'm 99% sure it will slide before it tips at any speed. The only way I'm going over is if there is a steep embankment involved.