the simple fact of the matter is that the saph has a great big lump of metal along the rear parcel shelf sits whereas there is a big hole and a bit of cardboard on the 3 door

forget about the rest of the geometry and stuff as if the shell is going to flex then you are onto a looser from the off, and stiffening the suspension will just make matters worse*

*these onions are based purely on having owned both a 3 door and a saph and the saph was easily the better/nicer car to drive on a day to day basis