How wide can you go in the rear?

[chris97boxster]

Stock 97 boxsters rims as seen above are as follows

Front 7J x17 ET 55 with 205/50 ZR17 tires
Rear 8.5J x ET 48 with 255/40 ZR tires

I would like to know also from other forums what they
have put on their 97.
yes I also understand that in the following years they
made improvents to the suspension etc for allowing larger wheels

18's are as follows for boxster
front 7.5J x 18 ET50 with 225/40ZR18
rear 9J x 18 ET52 with 265/35ZR18

I've got my eye on a set of Carrera III wheels that I would love to put on
but the rears are 11 inches...hence I have the same question as truegearhead
thanks
Chris