Nader,
I did some trial runs with a set of 18x7.5's I have (Ray's/Volk wheels, 38mm offset, 16 lbs each) and they fit fine but with 225x40x18 Dunlop Star Specs the range was impacted significantly; too much for my liking (10-12% at least). I ran them for a couple days and went back to my 16x7" set-up I originally posted about.
With going lower soon I was wanting to change to a larger wheel for the appearance. With that impact in range I will not be doing that unless we can get those Dunlop Ecosmart tires (215x45x18) that I saw one of the Japanese guys running (white car with gold Rays CE28's). A proper size low rolling resistance tire will be necessary to maintain range.