I seriously doubt that increasing the backpressure by sucha miniscule amount durning low rpm is going to cause a cat to die any quicker than normal wear and tear. One flap at the end of whole exhaust system is not going to make your backpressure change significantly. If you want to talk about back pressure making a difference remove your cat and run a strait pipe. You will lose some back pressure but your turbo will spool much quicker.
The only negative thing i've have read through this thread is possibly what Dailo posted about the computer constantly recalulation for the back pressure loss. Then again i dont know why it didnt just adapt to it. What if your driving wot all the time? It should stay open. lol.