Unfortunately, the horsepower number doesn't exactly relate to performance at that RPM as the figure has only a computational relationship with the RPM itself. The torque is the real power figure here. As in your graph, 5640 is about the area where your power starts to fall off.

For example, assume you're actually racing in the same environment, but have actual load applied. Where the car is pushing through all resistance and torque falls off, the car is no longer accelerating at the same rate.

Essentially, as those have said earlier, the extra revs are a point of diminishing returns.