Questions about new Cats

[Larz]

I just barely passed 100k miles and I have a code that indicates a failed oxygen sensor. It's covered under my warranty, but the car is also reporting cats not at effective levels. My warranty is amazingly generous compared to the Infiniti ext warranty, but I doubt any warranty covers exhaust or emissions parts. My service rep Geoffrey advised me about this last Wed and tried to resolve it before he left for London that night for holiday. He scheduled me for a drop off his first day back at work (this Friday). He was unable to get hold of my warranty company before he left. I called them and could not get a definitive answer aside from "no coverage for exhaust pipes and mufflers".

1) I am not interested in deleting my CATs, but only because I have no idea what the results will be or if it is legal to do so. I live in a non-inspection state.
2) If the CATs are covered by warranty, it will be for OEM replacement only and that will be OK with me.

BUT, assuming the worst and I need to pay for them, here is where I am looking for opinions:

Is there any clear advantage to installing hi-flow CATs on the 3.5L engine?
Is there an recommended non-OEM brand that I should consider to avoid the HUGE mark-up from the dealer?
Are there any danger of ill-effects form the sensors using non-OEM or hi-flow CATs?

I'm hoping that members with knowledge of CATs or experience with replacing them will chime in.

[Ilya]

I'd start with the O2 sensor and see if the cat code comes back...it's likely related. But that's my quick take on it so I could be wrong.