Intriguing - thanks for the link!
I have a few new bits on the way that will probably cure what ails me. I will of course post my results here. Thanks again to Todd and his crew - they have been extremely helpful which is all the more touching considering I'm a second hand, indirect customer.
FWIW, I tried a little experiment the other day. As many of you know, the tuning flap in the rear most crossover pipe closes at aprox 3100rpm, and opens again north of 5K. The more I paid attention to how and when the exhaust note changed at 3K (the rattle/rasp/awfulness I've been referring to), the more I noticed that it was a very sudden change - as in, almost instant...not a ramp up.
For kicks, I unplugged the changover solenoid that controls the tuning flap and eureka - the sound dissapeared. It's still not pleasant, but it's a linear tonal quality now with no sudden, dissapointing note change. This isn't a fix, merely information that to me anyhow demonstrates just how much Porsche tunes their exhausts to sound a certain way over the entire spectrum. With the OEM mufflers, the tuning flap changeover produces very little to no audible change in the exhaust note - the flow rate being what it is, Porsche have evidently tuned things to prevent this transition from sounding, well, un-Porsche like.
With the less restriction via the new cats, the changeover is no longer masked or tuned out as much and you get a most displeasing tone.
Anyhow, plugged it back in for the time being as the DME is obviously not keen to it being unplugged (no CEL, but a soft code observed via Durametrics). I imagine that Porsche tweak fuel and timing at this changeover and with the flap deactivated, the car ran fine but surely not ideally. At least it's an easy tweak if the sound drives me bonkers in the next little bit, but my new exhaust bits should be here in the coming days and this will all be an interesting experiment with something new learned.