There is a slight stretch with a 255 on a 9" rim, with a 10" rim it will really be stretched. A 10" rim really should have a 275/30/19 on it up front and do a 295/30/19 out back with the 11" rim.
Here's my ZCP car with 255/35/19 front and 285/30/19 rear. I tried to find a photo that showed the stretch already on the front just a 9" wide rim. A 10" wide rim with my 285s is somewhat stretched.
As far as offsets go with a ET25 front and probably wouldn't go much beyond a 31-33mm rear.