If the rolling radius is the same all round, you shouldn't have any issues. Keep an eye on the angle of the drive shafts if you are going lower and wider as there may be a risk of putting more stress on them than is needed.
You will notice a problem straight away as you will literally hear the haldex growling at you
But, if the RR is the same all round (means good math on the tyres calculation etc) and a good geo setup id say you will be fine.
For me, personally, I dont see the point in it - the car puts most the power to the front wheels, so its not even as if you are on the limits of rear end grip anyway.
If you are going for the wheels I think you are, why not try and get them all in the 8.5 width?