He might get a bounce but it wouldn't be a huge surprise if he didn't. Depending on the poll, a HUGE percentage of likely voters have already made up their minds. There aren't that many fence-sitters to get a bounce with.
I personally believe Bush will get a better-than-expected bounce. He will talk unfiltered to the American people, and he will remind them of his values and policies. Without the MSM telling you 24/7 why you should hate the president, he gets a lift.
Same thing happens with every major speech he gives, such as State of the Union addresses.