Matt,
I had the exact fault come up on my 2003 Corolla (Pre IS250) and the Totyota dealer was happy to clear it but said that it would most likely come back as it showed the cat was 'on it's way out.' (Just for info they gave me a quote for around £1200 to have a new one fitted!!)
After having it cleared I was happily driving away, but then after about 100 miles or so the Engine Management Light returned. So I had it checked to see what the fault was this time and it was indeed the same Cat efficiency fault.
My point is, even though you have had it reset it may still return. I thought of two things when the light came on:
* If the fault is detecting a discrepancy between the pre and post cat sensor, how do they know it isn't the sensors that are faulty as opposed to the catalyst itself (Obviously
they may want to assume worst case so they get the most of you)
* Secondly, my fault apeared quite soon after I had my M.O.T. done. I thought, if the cat was on the way out then the emissions test must have been close to failing. Checked
it and it had passed with flying colours.
After having the light reset a second time I'd sold the car to purchase the IS250 I have now, so I can't say how it's acting now.
Just beacuse the light comes on though, I don't like the fact that they suggest replacing the whole cat straight away, when it's possible it may be a faulty sensor. (I design the equivalent of cat converters for aircraft for a living so I'd like to think I know what I'm talking about!)
Hope that helps mate :)