There was a discussion on this earlier on. It depends on what is the cause of the P0420/P0430. I think sometimes, the codes will cause the PCM to go into the limp mode (more gas is used) and sometimes no. In any case, any CEL codes should always be taken care of ASAP.

My present mileage 18 City, 24 Highway (65 to 75 MPH), and recently 26 Highway (55 to 60 MPH, did it just to test).

Edit. Here is the thread with a discussion on it.

"Always do the cheap and easy ones first." 1996 V6 ATX 96K miles