when the turbos are spooling up based on it having the wastegate gases no longer impeding the turbine gases - the wastegates are always shut during spoolup. But, the fact that there is a laminar flow of gases out of the back of the turbine as they shoot into a same-diameter pipe which is mandrel bent will reduce the backpressure AFTER the turbine. This enhances the absorption of exhaust energy into the turbine which therefore makes it spool faster.
*This is all theory - the quantitative results, such as ACTUAL pressure/flow measurements to compare the two configurations with would be the ideal way of proving/disproving anything. Otherwise, its just talk. :)