Hi,
The Boxster RollBar is made of high tensile strength Stainless Steel. It should provide more than adequate rollover protection. That said, a rollbar extension is required in some forms of racing and DE as a Tech Spec.
For information on Boxster safety in general, and rollbar safety in particular (page 6 of the report), see Porsche's report to the NHTS here:
http://www-nrd.nhtsa.dot.gov/pdf/nrd-01/Esv/esv16/98S8O06.PDF Hope this helps...
Happy Motoring!... Jim'99