Alright so there seems to be some distinct mechanical differences between GR86 and BRZ
This is what I can understand from this vid but even if you don't they compare the two directly with a car cross section. Sorry if I'm not using the right terms:
Front suspension knuckle - BRZ is aluminium, GR86 is iron
Front stabilizer bar - BRZ is 18.0mm hollow, GR86 is 18.3mm solid
Rear stabilizer - BRZ is 14mm solid but has an additional stabilizer bar, GR86 is 15mm solid.