Here it is, from TSB #4407, dated March 4, 1997: " Eighteen inch wheels are not approved for use on Boxsters. Use of eighteen inch wheels on Boxsters under severe conditions may result in structural failures of the body and/or suspension".
I guess the key here is "under severe conditions".
This caution was removed when the frame was reinforced in later years, but I guess that must be in another TSB.
Regards, Maurice.