So I went back to it and decided to try buying SH instead of "shorting" SPX, and it gets even worse.
On 9-30-2008 the open prices were
SPX=1,113.78 SPY=113.51 SH=75.15
On 3-5-2010
SPX=1,125.12 SPY=113.37 SH=51.37
SPX and SPY roughly track each other, but SH is...