You will have lots of trouble with wheels hitting guards, not sure about the handling benefits but the GT-R skylines are wider than GTS-T in the rear for that reason but the guards are also pumped in the factory.
Cars generally handle better with a wider front track, and most cars are made this way to my understanding.
So if you widen the rear, widen the front aswell!
ive been having a look at a few cars (toranas commodores etc) and running a string line from front wheel to back wheel, they sit at the same width anyway?
widening the rear track will make the car more stable in a straigt line.
i put a vl diff in my vh ( wider than orig ) and it lost its darty go-kart like turn in on the corners.