From what I have read the only ones that have done real world testing with the stock cats on the 991 Turbo is Evolution Motorsports....

"We found much different behavior from the 991TT stock cats than we have on previous turbo models. We did not find huge power gains were had by going from stock to high flow cats, and our earliest test cars have not had any issues after 9-10 months of being beat on through half mile events, track days, and normal 911 Turbo daily usage. That being said, if we were to build a car we knew was going to be heavily tracked, we would recommend high flow cats for reliability until we get a better idea of how the stock ones will hold up long term."

Has anyone else done actual testing with the stock cats on the 991 turbo or is just an assumption that we are all using that the cats must be changed out?

 

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My car had them on it in the FVD sport exhaust when I got it. However, I put them on my m5 and I bought expensive Noelle downpipes to get the best ones (HJS). I think they are worth it because they allow the tune to get more from the car and they sound better but don't stink like catless. In addition, no CEL and I can't have that where I live because of emissions testing. If my car had not come with the exhaust, I would have added it and the high flow cats. I also added headers which seem to be at issue here on the forum as to whether they add to or subtract from the performance of the car.