The Mafia II engine passes physics handling over to the NVIDIA PhysX libraries and takes advantage to offer high physics settings which can be partially accellerated by the GeForces.
To measure performances we used the built-in benchmarks and all graphics options were pushed to a maximum, first without activating PhysX effects accelerated by the GPU:
Here the Radeon HD 6790 only has a very slight advantage over the Radeon HD 5770 and GeForce GTX 550 Ti.
Next, we set all PhysX options to high:
With PhysX effects pushed to a maximum, performance levels dive. Note that they are in part limited by the CPU, as not all additional PhysX effects are accelerated. Of course the Radeons remain a long way behind.