You need to watch to make sure that the TIRE DIAMETERS remain as close as possible (I would think less than 1%, maybe some can chime in on this figure)to one another (front to rear) to prevent damage to your awd (xi) system.
The chart above allows you to check for this. Plug in the tire sizes your considering in the original/new tire boxes and check to what the difference is to one another.
The sizes you listed are 4.28% difference in diameter to one another.
Also tire sizes will dictate rim width. If you do not want to run a "square" setup (same rim diameter and width at all positions) you will have to run staggered profiles (i.e. 225/40-18 front / 255/35-18 rear) to run different width tires.
Exercise caution when going to the 40, 35, and 30 series tires, pot holes, curbs, and railroad crossings can play havoc with tires/rims as far as possible damage.
Check the All-Wheel-Drive (Xi / xDrive) Talk forum, have seen threads concerning both staggered setup and lowering the xi's
http://www.e90post.com/forums/forumdisplay.php?f=197