The Dow is at a six year high...yes, investors are abandoning the GOP in droves.
The false premise is that any drop in approval of Bush amongst investors would be due to Bush's policies regarding their investments. A group called "investors" has many concerns other than their investments. If I owned stock in a company that hired illegal aliens, for instance, I wouldn't necessarily judge a President solely on how good he has been about allowing illegal slave labor even though that slave labor meant more company profits and therefore increased my wealth. Then there are those investors who might see a future where the Democrats are more friendly than Republicans towards the usage of illegal slave labor and suddenly prefer Democrats for that reason.
Adjusted for inflation, the Dow is about 2000 points below where it was 6 years ago.