ValidusVentus

My 2c:

-Check to make sure you have at least .15 degrees toe in per wheel in the rear. If not, make that happen.
-Try to ensure that you aren't overdriving the rear tires and getting them greasy(er) than the fronts. Lowering your rear pressure will help on grip and make them a little warmer but its still possible to overdrive them and still get them too hot. if you raise the pressure back there, as long as you aren't overdriving them they should stay a bit cooler, but it will lower their grip a bit and make it easier to overdrive them.
-I don't recommend swapping back to xp8s all around since that pad is not nearly heat tolerant enough for our heavy cars front brakes if you use them near their abilities. Furthermore, a swap to xp8 in the front from xp10s would effectively alter your brake bias more towards the rear. I don't see that as being a good thing for stability under braking if you are already having issues.
-Maybe try being slightly slower on your application and release of the brakes.

All that being said I think all the slush in the rear stock suspension between the rubber bushings and fluid filled bushings contributes to this. Also as the suspension unloads in the rear and goes into droop I believe the suspension geometry starts to generate toe out, causing instability, well that and the fact that the wheels are pulling back pretty hard on all those flexible bushings while braking, adding more toe out. Which may be why having stiffer springs in front seems to mitigate this some. It doesn't change the amount of weight transfer but it does reduce the amount of suspension movement in droop in the rear.

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