^Good question, it was brought up here
Wider tires on sport rims? and also look further into that thread for some additional feed back.
IMHO I would prolly stick with 20mm rr wheel spacers if I was planning on over sizing the rear tire on replacement to anything wider than the 275 stockers.
Other members that are more knowledgeable in this area (RCZ and Mike) than me and whom I believe are running over size tires on their stock 19" sport rims and also have wheel spacers may want to chime in.