I'm running 275's all the way around, front and back. It has a tendancey to "jog" a little bit. I have a bump steer kit and and koni adjustables that help with that. Just makes it easier to rotate them if you know what I mean!!!
On 275's on a stock rim is as big as you would want to go. If you go any bigger, you are wasting your money because only the inside of your tire will make contact with the road. It would be like a bubble in the center, not good for traction. I am thinking about going to a 285 in the rear as I have 17x9's on all the way around to clear the brembos!!!