If you already have 245 on 15x7 then should have fairly good idea how 255 will look. Probably on the limit between a good fit, not too bulgy and handling.
Also, 255 on the front brings it close to fender edge. That can look weird on some when same size is tucked in at the back. You could get just 2 fitted if unsure, check clearance on the front, then on the back, and then on same side. If a happy chappy...go for it.