Jeff,
A downpipe does increase efficiency of the turbo, however the problem is when you increase the efficiency of a small turbo such as this one you make it "run out of steam" easier. HP is a calculating based off of torque. (Torque x RPM / 5252). When you add a downpipe the torque comes on faster but also plummets faster, and when torque drops so does horsepower. You will see a small increase over the stock downpipe in the lower rpm range, but mostly on lower boost (21-23psi), definitely not on higher boost or PEAK power which is measured at 5252rpms.
Restriction produces torque, as well as many other things. When you remove restriction you lose torque, that is why the stock downpipe shines and provides more peak gains than with a downpipe as the restriction helps hold torque through the rpm range. This is why when some tuners don't properly tune a car and the vehicle has throttle closures it produces a ridiculous amount of torque. The throttle blade closing is a restriction in the system. When I first started tuning Ecoboost vehicles I noticed there was always more power and torque when there were throttle closures, however it causes issues for many other calculations and isn't safe.
For me, more power in the upper rpm is more ideal as when you are rowing through the gears you aren't starting at 1500rpms in each gear. So anyway to keep torque up through the rpm range is what I would prefer, and recommend to do.
