Have the 225/40/18 removed from the rear wheels (18 x 10) and have them mounted on the fronts (18 x 8.5). Buy new 255/35/18 and have them mounted on the rears.
This would be the stock sizes supplied by BMW, Results in 0.236% difference in diameters front to rear. (that is less than 1/4%)
18 x 8.5 is 1/2" wider than BMW stock front wheel, (18 x 8 is stock size) may provide a small amount of stretch.
18 x 10 is 1-1/2" wider than BMW stock rear wheel, (18 x 8.5 is stock size) will probably provide some stretch.
Try the following link for the "xi" forums, seems to be a lot of info also.
http://www.e90post.com/forums/forumdisplay.php?f=197