Performance comparison of "$BID indicator", Sotheby's vs. the $EEM (MSCI Emerging Markets) going back to 2003. You can see the close correlation throughout in this weekly chart.
Check out a larger image of the same chart here.
You can also see a 3 year performance view of Sotheby's vs. $ACWI (MSCI World stocks ETF) here on Chart.ly.
As you'll see, the correlation between $BID and $ACWI is not as close, and Sotheby's easily outperformed the global shares ETF over this recent 3 year period.