255/45/17 came stock on 94/95 Cobras, they fit totally fine on 17x8s. I would not go bigger really because you will have to go down in sidewall height and there is also really no need.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.