I don't know the exact difference but I assume the upper and lower arms on the 88/96 models are longer by around 18mm per side, but that may be too simplistic an assumption.
84/87 16x9.5" wheels have a +38mm offset while 17x9.5" 88/96 wheels have a +56mm offset, an 18mm difference, yet there is no difference in the width of any of these cars (except ZR1, but that's another story).