i agree, jonB seems to be the cat expert around here.
Long Answer:
The terms "ceramic" and "metallic" refer to the substrate of a
converter, and substrate material has almost nothing to do with the
efficiency of a converter. The process that "cleans" exhaust gases
(converts HC, CO and NOx to carbon dioxide and water) occurs because of
the precious metals (platinum, palladium, rhodium) and other compounds
contained in the coating that's applied to a substrate. If you coat a
metallic and a ceramic substrate with the same formulation, there should
be little if any difference in the efficiency with which the converter
operates-- assuming the number of cells and substrate length are the
same.
One potential performance difference is the effect of air flow
rate. Metallic converters have thinner walls than ceramics, so all
things being equal, a metallic will offer higher flow rates. That being
the case, exhaust gases will pass through a metallic substrate faster,
so the potential does exist for less efficient conversion than would be
experienced with a ceramic converter. However, at low engine speeds,
exhaust flow volume and velocity is so low, the flow capacity of a
converter has virtually no effect.
The advantage of a metallic substrate
is at high engine speeds where flow capacity does make a difference with
respect to power output.
Keep in mind that you can't lump all ceramic converters or all metallic
converters together any more than you can lump all Vipers together.
Obviously, a Viper with a supercharged engine will have different
performance characteristics than one with a naturally aspirated engine.
Similarly, the chemistry (amount of precious metals) of converter
coatings makes a significant difference in their performance.
Many of
the cheaper converters as made in China have the absolute minimum amount of precious
metals, so they're marginally effective for a relatively short period of
time. Please: Know who makes your converter, and where!
As for the rear oxygen sensor, it has no EFFECT on converter operation
or engine operation for that matter. Its only function is to monitor
converter operation.