Yes of course I realise this, but your front brakes do the majority of the braking, when your slowing down the weight is thrown to the front of the car, ever realise why on pretty much all cars the front brakes are bigger/better?
And the weight is transfered to the front of the car because....
The grip from the tyres to the tarmac causes the front suspension to compress moving the weight distribution from the perfect 50/50 BMW weight distribution to being more like 80/20 front rear (guess!).
However if its icy as **** then you won't get anywhere near like the same amount of weight transfer due to reduced grip/roll/etc etc, so im sure the rear brakes will slow you just fine.
If I had one set of chains, id be putting them on the driven wheels.