[woodsman]

I don't think wheel size in the 17- 18 range matters much. It's the springs that you're feeling. If you could swap in base Boxster springs front and rear you would have coosh. The tire sidewall ratio matters next. This is expressed as a percentage of the tire width. You may have a 35 aspect ratio and increasing that to 50 or thereabouts would soften things up. Maybe you could swap your struts with some else. Either way the car can be softened up a large amount. The rear matters most but it's a system you don't want to upset by doing one end only.