Here are some pics of rollovers. Of note is "No Injuries". These cars actually have as strong or stronger roof structures via front cowl and roll bars as any car out today.
http://www.wreckedexotics.com/boxster/boxster_121102_005.shtml
Car:
Porsche Boxster
Description:
Driver Comments: "These pictures were from my accident on July 4 2002 in my 2002 Porsche Boxster. My girlfriend and I were going to my parents house out in the country. As usual I was pushing the car to the limits. When going around a corner we lost control, spun out, went through the road, up an embankment and then kicked back out to the road upside down."
Injury Report:
No injuries
Note the condition of the windshield here:
http://www.wreckedexotics.com/boxster/boxster_20030820_001.shtml
Look at ragtop here as well:
http://www.wreckedexotics.com/boxster/boxster_20030820_002.shtml
Car:
Porsche Boxster
Description:
Rolled 2-3 times
More info on chassis materials of construction are here:
http://www.porsche.com/filestore.aspx/normal.jpg?pool=usa&type=image&id=bo-detail-safety-passivesafety-xl&lang=none&filetype=normal
Also look here under "Passive Safety":
http://www.porsche.com/usa/models/boxster/boxster/indetail/safety/