For a Canadian-equipped vehicle, it's 60 kg (130 lbs) heavier. Not 300 lbs. And that's very low-slung weight as well.
The comments about increased understeer are being bandished without any qualification. Increased understeer is possible in a very limited set of circumstances, but most times not. Such as if you're plowing the front end through a steep corner on full throttle, and the outside wheel has already lost traction.
Here's some marketing speak for the bored:
The BMW xDrive system detects any oversteering or understeering before they even start. Within a tenth of a second it distributes up to 100% of the engine power to the front or rear axle via the transmission and an electronically controlled multiple-plate clutch before returning to the normal 40:60 distribution ratio.
With the BMW xDrive system each axle uses its traction to the maximum. Whether in tight corners, during hill starts, on slippery surfaces through rain and leaves or on snow-covered carriageways, the optimum amount of power reaches the road. Your BMW provides stable acceleration in every situation – even out of bends. With BMW xDrive your vehicle exploits the benefits of rear-wheel drive to the full: precise handling, optimum cornering and a clear separation between the engine and steering.
and from the international site:
xDrive is the permanent all-wheel drive system from BMW: under normal circumstances, it distributes driver power between the front and rear axles in a 40:60 ratio, and changes this figure variably when the road surface or overall driving conditions change.
Acting virtually instantaneously and a manner so subtle as to be go virtually unnoticed by the vehicle's occupants, xDrive can direct up to 100% of drive forces to one axle. Enabling the driver to start up effortlessly even on slippery surfaces or steep hills, xDrive routes all power to the axles with the greatest traction. When parking, the system reacts to the need for high manoeuvrability at low speed by opening the clutch completely so the powertrain functions optimally.
At the first sign of understeering, drive power to the front axle is reduced. If oversteering is detected, xDrive directs more power to the front axle. Thanks to this dynamic redistribution of power, vehicle stability returns to normal even before the driver notices anything amiss.
Driving on a winding road or taking a fast bend in dynamic style is particularly enjoyable with xDrive: you feel as if your BMW is being guided along the curve. xDrive ensures that none of drive power is wasted on a loss of traction: every kilowatt of power is effectively brought to bear on the road.
xDrive is regulated by Dynamic Stability Control (DSC) and uses information from the latter system’s sensors to monitor road conditions. In addition, brake force courtesy of DSC is used when there is traction difference between the two sides of the vehicle and wheel spin is likely.
So basically as soon as it detects understeer, it diverts power (up to 100%) to the rear axle, and the resultant behaviour is thus quite similar to the RWD.