Firstly in regards to braking:
The amount of force provided by the contact between 2 surfaces is given by:
F = uN (fun) where N is the force reacting to the contact (newtons 3rd law) and u is the coefficient of friction (this depends on surface type and tyre compound/temp)
Lets do an example. say we have 2 cars, both way 1000kgs, one experiences 85% of the total weight under heavy braking on the front wheels, the other 65%.
We will take u to be 1.2 (average coefficient for a road tyre just before slip).
And gravity to be 10 m/s2 (because im lazy lol, real figure is 9.81)
Mass x acceleration due to gravity = Normal force (N) x u = force due to friction
car 1: 85% of 1000kg is 850kg 850kg X 10m/s2 = 8500N x 1.2 = 10200N
Which means the rear of the car is experiencing,
15% of 1000 is 150 150 x 10 = 1500 x 1.2 = 1800N
Total Force = 12000N
Big difference
car 2 : 65% of 1000kg is 650kg 650kg x 10m/s2 = 6500N x 1.2 = 7800 N
and the rear = 4200 N
Total force = 12000N
Now what this shows is that on the FF car there is a lot more force put on the front wheels, which means an increase in the amount of braking force that can be applied.
This however does not meant that it can have more braking force applied overall.
This is why FF/FR cars have big front brakes and smaller rears, to take advantage of the weight shift.
But in the MR2 the brakes will be much more evenly sized.
In summary: Weight distrubution does not affect the total braking performance overall, but it does change the way the system must be designed.
Hope I havn't bored you all to death. But this was a great distraction from study for stupid renewable energy systems, bleh.