A local LA shop has this posted on their website:
"Most extreme fit 20x9 et50mm and 20x10 et 60mm(15mm spacer) with 245/30/20 and 285/20/20"
They also have a picture of 20x9et 50mm and 20x10et 60mm wheels on a Boxster
Scroll down using the link below to find the Boxster fitment:
http://www.wheeldynamics.com/appguide.html