I get those codes every once in a while, and when I monitor the durametric I do not have a flat line on the post cat O2 sensors, it is wavy and trails behind the wavy line on the precat O2 sensors. Mine are definitely near the end of their life (2000S 115k miles). But, I don't get the CEL and those codes unless something else is also going on. I figure mine are wearing out, and right near the end of their life, but they are still within spec as long as everything is right. On my passenger side I was getting that code, in conjunction with 2 or 3 other codes (I can't remember what they were, maybe 1128/1130) but one of them was an aging post cat O2 sensor, and also the heater for that sensor had failed. Replacing that sensor cleared up that problem, but I also did some other work at that same time, tightened all clamps in the engine compartment and did spark plugs/orings). That code hasn't come back on that side. (The front O2 passenger sensor is relatively new)
Last year I got the driver side cat code on the way to an autocross. Cleared it and it stayed gone for a while, then came back one or two more times, and then it came back along with a code on the front driver side O2 sensor (aging or heater code, can't remember). Replaced that, and that code hasn't come back.
Interestingly I never get these codes when I am at the track, it is always puttering around town...
I know my cats are at end of life, and I have a set of takeoffs I got that have only 20k on them (or maybe 40k), but I am going to squeeze every bit of life out of the ones I have
Very short version: Fix/resolve all other codes/issues first and see if the 420/430 resolve themselves. Sometimes it does and you can squeak a little bit more life out of them.
Also on Boxster register there was a guy that had a lot of trouble with those codes and it turned out his Alternator was failing and the sensors and computer didn't have enough juice to properly run so he was getting those codes. New alternator solved it for him.