The Wren pipe works incredibly well and produces additional thrust while allowing air to move freely from the efflux of the turbine, thus helping to keep the EGT down slightly.

As far as I know, the design details are not patented and the 'how to' details were published in RCJI so, it is in the public domain.
I am also amazed that others have not adopted this pipe, it works for sure and can be proven on a test stand.

As for bypass, I have used them and if you want extra top end then they are a must due to reducing the internal drag on the model.
However, for most 'sport' applications and sensible installations (keeping as much out of the airflow as possible), I have found them to be unnecessary.

Hope that helps.
Paul