According to the fabspeed dyno graphs, the secondary cat bypass adds 50 hp to the wheels on F90 and does not require a tune, since secondaries do not have O2 sensors in them. How true is that? Seems like a crazy gain from just a secondaries delete... I had an impression that such power increase is only possible with primary cat delete??
https://www.fabspeed.com/bmw-m5-f90-...-bypass-pipes/