Don't know about the spring rates but I've read that the turbos do have beefier antiroll bars. Some people have installed them on their non turbo cars to get better handling and flatter cornering. Manufacturers tend to assume that people buying turbos are performance minded and desire a better handling car. In the age before dynamic computer controlled suspension tuning it was an all of nothing sort of thing. Couldn't have soft cushy ride and good handing at the same time.