IMO I think that it would be a bit overkill to be honest. The fronts should always be more powerful than the rears, so you would have to go HUGE on the front to redress the balance of braking force. The bias valve would send a proportion of braking to one end or the other, but the stopping power the 4 pots have got is going to be enough to lock them up even with a lower split of braking. Bear in mind there's next to no weight in the back of the car compared to the front, and when you do brake the weight will effectively shift forward and exaggerate this in-balance.

At least that's how I see it, but I could be wrong. I'd agree with the above comments and upgrade the fronts. Just going to vented or a slightly larger diameter disc on the rear, would give an increase above the effect of the same upgrade on the front, due to the difference in weight and the transfer effect, if you see what I mean. As said, all IMO.

cheeRS