215/40/18 will be expensive as they are not a commonly stocked size (or so I was told when I tried to order some). As mentioned, the tyre will also look stretched across the rim leaving you more open to curbing your alloy. That size would most likely sort your rubbing problem though.
I had 215/40/18 on my old Legacy and it never rubbed even with passengers in the back. When it came to getting new tyres I went for 225/40/18 and sure enough rubbed every time I turned a corner, went over a speed bump. Even worse with people in the back.