I have 275/40/17's on the rear. I had them on the front too, but didn't like them, so I put 255/40/17's up front. I like this combination. Gives the car a slight "racked" stance and drives good too.
I have 275/40's all around. My thought is, if it's good enough for a Cobra it's good enough for my GT. It does track a bit on crappy roads but I've grown accustom to it. I had stock wheels back on my car for a week when one of my tires was messed and I had to order a new one and it was so weird not having it pull it was nice to get back. As weird as that may sound!