You'll only ever notice a difference whether it's there or not if it rattles.
So remove them from each cat.
Easy
Nick . . . this "cat rattle" on my 2.6 I have a loudish rattle only when the car is started from cold , meaning overnight cold. On cold start up it rattles like hell for about 20 secs then fades completely after about a minute or 2. This is coming directly from the drivers side cat from the heatshield area.
I might try & remove the heat shield as it must be this that is causing the rattle. I'm guessing as the cat heats up something must be expanding & the rattle stops.
To remove . . . is it a case of just carefully grinding through the welds & the shield will then come away?
Or am I getting this totally wrong & its something in the cat rattling?