> In your opinion what would be the effect of installing a new cat: on a)
> the sensor readouts and b) fuel consumption? (in other words does fuel
> mixture need to be readjusted after a new cat is installed?) DC

Quite simply, no.  Since air mixture tuning via the O2 sensor is done BEFORE the 
converter, a new or old converter will make no difference in sensor 
readings.  Now for the qualifier: If the old converter was carbon blocked
 or the element broken out (causing blockage) and someone had subsequently 
adjusted CO to compensate for this, then a re-adjustment would be 
necessary.

A blocked converter will definately affect performance and fuel 
mileage, with a new one being the cure.