The Power Flow exhaust is standard fit on the Diamond DA40 for a few years now, so they must have convinced Diamond it's a worthwhile power gain
...or they simply got a good deal.
anyone claiming large performance gains by fitting a tuned length exhaust to a Lycoming or continental four stroke is telling porkies
As the bloke above mentioned, exhaust tuning is 2 stroke territory.
Take a look at a merlin or griffon engine, these engines were optimized for power, they have simple stub exhausts, the only time anything more fancy was fitted was to surpress exhaust flames and these always gave a performance loss