Dave,
It is the nature of this turbo to produce the torque curve like this. A standard T3/4 is much more agressive in the way it comes in with a bang (normally 400lb ft at 3500rpm). However, not only is this horribly agressive and a killer for gearboxes in a 4x4, the standard T3/4 then tails off a lot more rapidly. This custom turbo, although it comes in less agressively (due to being bigger), it can hold onto the torque / power better. This allows the boost curve to be mapped nicely to give a lovely flat torque curve that will make it feel awesome in the car and make it feel like the power is still there all the way to the limiter, where on a normal T3/4 car, you get a huge mid-range wallop, followed by the feeling the power is tailing off at the top end.

As a comparison, so you can see what I mean, here is a power-plot for a standard T3/4.