The motion ratio in the rear is 0.64. Square it and multiple by the spring rate to find the wheel rate. The shock mount has a motion ratio of 1.02...
950lb springs in the rear comes out to 390lb at the wheel. A 400lb coilover setup is stiffer then the 950lb spring in the stock location.
I'd put driver ability above even tire selection. I had fun 2 seasons ago with the stock 17 year old suspension beating up on cars with slicks and built suspensions in my class. And I'm not a good driver by any means...
FWIW, a stock 318ti will 2-wheel on 215 wide R-S3 tires.
