Run 275s in the front for a 9" rim, and non nitto 315s in the back on the 10.5" rim. the nitto 315s are narrower than a standard 315 and will look like ***.
To figure out the height/profile of a tire, there is a formula. The height of the sidewall is the first number times the second number as a percent. If you have a 275 35 R17 tire, you would take 275 and multiply it by 35%...that will give you the sidewall height in millimeters. So, a 275 will give you a sidewall height of 96.25 mm, or roughly 3.85". A 265 35 will give you a sidewall height of 92.75 mm, or roughly 3.71". As you can see, there really is no noticible difference unless you can see a 0.14" size difference.
As for benefits of getting the massive meats in the back, there will be no real performance gains. As brad said, you will probably slow down a bit as you will be turning a lot more standing weight from the larger tire as well as the larger rim. You will, however, gain all the traction you will ever need in near stock form. The odds of breaking loose are slim to none, unless it is cold out and your tire is wet.
There used to be an article about tire and rim sizes. Not sure where it went, but maybe brent or spector can answer that question. It used to be here www.mustangevolution.com/articles/wheelsandtires
None the less, go with 275s in the front and 315s in the back. Those are the same specs I am running on my car. The fronts are 17x9 rims, and the 275s fit just right. A 265 will be too thin and not look as good and risk curb rash. The backs are 17x9 rims as well, with a 315 nitton on them. It works because of the width difference with nittos. A BFG or mickey tompson will fit the 10.5" rim as a 315...the nittos won't.
In all honesty though, I think you should save yourself some money and get 17x9 rims all the way around. Then you can still run a 315 if you choose (nitto) or get the 275s or 285s for a slightly wider look. This will let you rotate your tires (assuming 275s all around) and save some money on rims and tires. The only 315s I think you will find will be in a drag radial, but I could be wrong as I have only look for DRs in that size in the past.