I know this is probably too late, but I have my 04 Matrix lowered 1.5" and I have 18x8 and there is plenty of room. I called VictoriaTires and Rims
http://www.victoriatire.com/ and they were very helpful. I do advise that you get wheels that are an inch bigger in the rear. With the way the Matrix backend is bulkier then the front, you need a bigger rim to fill the rear fender. I have 18s and I know its an illusion, but the front rim sometime looks bigger then the rear. I would go with 18x8s in the front and 19x8.5s in the back. Hope that helps!!!