How wide of a tire can i put on a 2004 mustang mach 1 17'' tire?

i think the rims are 17x8. the tires from the factory came with 245/45/17. I was looking at some 275/40/17 but the "approved rim width" was for 9'' to 11'' width wheels. Does anyone have 275 or greater on an 8'' width wheel? Is that safe? How wide can I go without having to worry about an accident if I were driving hard?