Sufi,
Thanks for the fot, makes the differences between SRT and NA clear. The secondary cats are much further back on an NA and no resonator.
Spoke to 2 Maganflow and 2 Powerflow exhaust shops here in London. All four said an 'X' pipe on my (NA) system (with sec cats and 'Y' pipe feeding single muffler) would be a waste of money.
I mentioned drone and none could comment , suggesting (like you) to see if there is any and how bad. Even though they had obvious benefits in selling/fitting an X pipe, none were keen to do it as they couldn't gurantee it would reduce/kill any drone.
One shop suggested they could install an 'X' pipe, and then feed the outputs to two separate mufflers, centrally side by side .
Still haven't had a chance to drive the car on the motorway between 50-60.
Mick