Baz, here are a few pics, hope this what you are after. There is still room in the guards for bigger, either diameter or profile as Craig said with the 325 45 17.
I have to agree with Craig and Dave, Billboards would be my choice if I was to go to 15's. However 18's may open up some more tyre options as well.
As Mike and Craig W mentioned, lowering will assist with reducing the gap. I have gone as far as I would go (see last pic).
