As Alex has said above, due to the mechanical limitations of Haldex, you cannot ever get more than a 50:50 split of drive front:rear. You will never be able to make the rear wheels rotate faster than the fronts when Haldex is engaged.
In some limited circumstances (front wheels up in the air, or front wheels on ice) - you can get
near 100% torque transfer to the rear axle, but this is more Haldex marketing than real world.
If you have a lot of lock on, and boot it out of a slippery junction, you can get power oversteer, same in the snow. But not on the move with high friction surfaces.
And then, even when you do get the torque transfer to the rear wheels, one spinning rear wheel will render the system useless (the new Focus RS uses a new GKN driveline system to get around this problem).
A limited slip diff in the front (like the AMG A45) is a far better solution to handling and traction, rather than chasing a quicker lock up time on the Haldex controller.
There are a few flat-earthers around who insist that Haldex is every bit as good as Torsen (proper quattro) but quite simply, Haldex is a compromised solution to enable transverse engine cars to have a form of (part time) AWD.
All this, and more, in the Haldex FAQ (including differences between Gen 4 and Gen 5):
http://www.audi-sport.net/xf/thread...-on-the-8v-chassis.198794/page-2#post-2286712