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.
1999 Techno Violet Metallic BMW M3
2005 Audi B6 S4 quattro 6MT
2004 BMW 330i
1986 BMW 535i