Hmm, the P0420 is quite well explained in the error codes document for the z22se (http://www.speedsterclub.nl/bibliotheek/technische%20documentatie/z22secodes.pdf). From what I understand, the timedifference in reaction between the first and second lambda sensor is too big when this error code is set. That would account for the description of the error, where it says that the oxygen storage amount it too large for the catalyst.
So basically this shouldn't have anything to do with the fuel trims the ECU uses, more the reaction of the first and second lambda sensors.
The P0430 is the same, but then for bank 2. Bank 2 refers to the second exhaust system. If for example you have a V6, bank 1 is the first row of 3 cylinders with their own cat and 2 lamda sensors, bank 2 is the second row.
As the Z22SE only has 1 bank, you should never get a P0430 I think. Unless you added some cylinders 😃
Mark