to take advantage of resonance...same idea, but you want the burnt gasses to move away from the turbine as fast as possible and not to return...and stagnation does nothing to help. You want your DP inlet to match the diameter of the turbine outlet and keep your waste gate exhaust flow separated as much as possible. You then want your pipe diameter to expand slightly, but not enough to stall flow due to back pressure. 3" Midpipes could be overkill for most set-ups as well.