In my experience, the Forza physics engine doesn't properly simulate driveline lash, in either FM4 or Horizon. Power oversteer is well accounted for, and you'll feel the difference between an open/limited-slip differential while spinning the wheels, but the drivetrain has little effect on handling in an off-throttle situation, rendering the 1.5- and 2-way options kinda pointless. Relative to the real world, all cars in FM4/Horizon are as steady as a rock when diving into a corner.
It helps when you can tune the differential to the extremes, but IMO minor tweaking has only a placebo effect.
