I do not have to pass emissions here, so the efficiency of my high flow cat isn't really measured.
My car does not use o2 sensors. It's tuned in open loop.
As for how that's done exactly I don't know. I'm no tuner.
The only o2 sensor in my entire exhaust is my wideband.
'06 forged, turbocharged, meth-fed Ford Focus. New 1/4 numbers soon.