17" Arc-8s don't look as good as the 18" variety, but they still look excellent. Everyone is correct in saying 17" tires will be cheaper. Definitely an advantage for someone who autocrosses. Have you looked into a set of 17" Kosei K1 racing wheels? They're inexpensive and lightweight. An autocross favorite.
Past: 1999 Techno Violet Metallic M3
Present: 2011 135i 6MT