Catalyst efficency below threshold

[Captncarb]

My A6 2.7T upgraded with K04's, 3 inch tapered to 2.5 downpipes, and high flow cats periodically sets the following codes: P0422 & P0432, according to the reader they are Catalyst Efficency Below Threshold bank 1 & bank 2. I'm being told it is my high flow cats, is there a way to correct the signal from the 02's so it will stop throwing these codes? The CEL always seems to come on after the car is warmed up, shut off for 1 to 2 hours and then started again after a few minutes of driving, it also is more persitant in hot weather verses cold. The car runs strong but fuel mileage is poor, I have an adjustable press regulator which if adjusted any leaner than current setting seems to trigger a fuel system too lean code at part throttling, mileage is still bad also at the leanest setting, the computer just seems to compensate.

[Captncarb]

On Ebay there are some extensions to put into an O2 bung to move the sensor out of the high flow behind a high flowing cat, no more codes now and I was able to set fuel pressure for best power. The fuel mileage is better but not great, I still use the stock chipping but intend to upgrade ASAP.