I'm no fan of DB killers, i wouldn't be surprised if they are the cause of damage to the carbon sleaved termis due to the excess back pressure they cause.
I'm my experience it's over rich fueling that does for a cat, they certainly don't like unburnt fuel on them at all.
Yep it's hassle swapping systems depending on noise limits, one reason why I don't do any quiet days!!
Maybe it;s worth looking into one of those new fangled Motty fueling thingies, it's reckoned to adjust fueling based on readings from a lambda sensor.
More info here http://www.afrtuner.com/description.html