If your going to run the same size all around I would at least keep with the factory size.
Only because on average the 245/45 will be 1.3% off. Your speedo would read 60mph but you will actually be going 59.2.
go to Tire size calculator plug in the stock size and you can play with the variables.
Also given the GXP has more torque, it was designed to have a wider tire up front, going smaller would be silly. Wider would be wiser as it would give you more traction, unless you want to be able to burn them off faster, then do as you please.
Again, I'd stick with stock sizes or push for the 275/40 for the fronts and a 245/45 on the back.
Also, again... your call. Price them and go from there.
Traction is based on weight (actual weight, not the misconception that weight is mass) and coefficient of friction which is based on tread compound and temperature.