If I remember my Bernoulli correctly, the constriction in the downpipe will reduce the pressure and draw exhaust gases through the turbo at low flow rates, maintaining rotation of the turbo and hence available boost pressure. The driver would see this as reduced turbo lag and good response to opening the throttle.
It will also reduce exhaust gas temperature which may have other benefits - increased longevity of the downpipe, control of the temperature at the cat, reduced heatsoak into the turbo housing. Your new downpipe will allow for greater bhp figures, but the original wasn't designed for ultimate power.
