By word of mouth, the Carrera 18" is the second lightest Boxster wheel - reportedly the new technos are lighter. Tire weight plays a big part in your total unsprung weight also.
per
http://www.europeanspeed.com/wheelweights.htm
Porsche Boxter Cast 17x7 20.3
Porsche Boxter Cast 17x8.5 22.7
Borbet Type C2T (Porsche OE) Cast 17x7.5 22.0
Borbet Type C2T (Porsche OE) Cast 17x9 23.5