I think if you want a stiffer rear setup its better to use stiffer bushings in the control arms.
First of all compression/expanding of those bushings has more effect on wheel geometry, and secondly if something in that department fails its much easier to repair. Just swap out cheap parts. Repair welding on chassis is much more expensive and its not always apparant where the damage sits.
Look at the youtube video's redish motorsport puts up. They specialize in the e46 chassis and know it by heart to the spotweld where some of the problem areas are and that often means cutting out whole panels to get to certain enclosed box sections where internal damage sits. With the z4 chassis.... no one has that experience and if something goes wrong in that department you can be chasing that problem for years, cutting the whole car apart panel by panel.
This video gives you an idea why you want to prevent damage to the body at any cost... The damage you see with your eye is only part of the story.
Its the main reason why all my polyurethane bushings that directly touch the body arent stiffer than shore80a