My guess is, you can forget what the door tells you. there's a guy on our S2000 forum that tows a 3000lb boat. with a stock S2000!
I guarantee you if that car can do it, a Boxster can. Though I wouldn't do it everyday.
http://i60.photobucket.com/albums/h2/suzukas2000/DSC00100.jpg
http://www.youtube.com/watch?v=notSrYYpObQ