No pics of them side by side but if I remember correctly before I had them mounted the 275's were maybe 1/2"-1" wider than the 245's. It's pretty noticeable but if your rims are wide enough to fit 315's, I agree with Cali03Boss that they look better than 275's.
You cant really notice that much of a difference. But like everyone else said, if you have big enough tires then get some 315's. Here are my 275 nittos. BTW, these tires are ****ing awesome.
Nope. With the right sidewall you could even do 335s. But i wouldn't might look funky. The only rubbing that might occur, is if your car is excessivly lowered below 2 inches in the back; when you go over a steep curb, the back tires could bump up against the fender. But over all, the 315s sit right up in line with the fender, and makes your car look like its about to choke a *****