There has been extended research done on this topic.
Had the same problem on my F430 spider.
Cutting the long story short, the cats do not produce enough back pressure so the lambda sensors pick a minimum reading and therefore you get a P0420 or a P0430 ECU code.
Solution, is either you go back to stock cats, or reprogram your ECU so the lambda sensors are disabled.
Yes you should be having a safety mode that reduces power slightly because the ECU thinks that the cats have failed.
And to save you time and money, these little gimmicks like mini cats and extenders are totally useless.