EVOMS

They gloss over it when they say "The signal of the oxygen sensor downstream of the catalytic converter is used to monitor the function of the catalytic converter, as well as to correct the oxygen sensing control".

I'm admittedly less familiar with how it is used on cars with narrowband oxygen sensors like the 996 Carrera and GT3, but on the GT2 and Turbo (and newer Carrera models) which use wideband primary oxygen sensors, it is important in keeping the sensors calibrated and compensating for their 'drift' as they age. A similar function occurs on the narrowband cars, with I believe partial throttle fuel trims being the primary impact.

This functionality exists for emissions reasons, and is there to make sure the car runs cleanly with 100k mile old sensors. That is likely why it can be removed with European coding (who have much looser emissions standards), but if it were me I would rather add a couple more O2 sensor bungs than remove features that optimize how my car performs. Call it a pain to deal with, but I'm a real believer that without ever-tightening emissions laws we would have a nasty environment and moreso we would be missing out on lots of cool technologies that not only reduce tailpipe emissions, but are making engines and cars more efficient and in turn more powerful than we could've dreamed of even 20 years ago. Things like variable valve timing, variable valve lift, downsized/turbocharged engines, internal engine coatings, fancy manifolds, low friction fluids, and passive or active aerodynamics are really things developed for mass consumption out of necessity to meet emissions that all just so happen to have major performance benefits to enthusiasts like us. Vorsprung durch Technik!

Sam