Muffler delete?

The best and non existent mod would be:
a custom turbocharger fitted and then an external wastegate.
Then the downpipe would have no turbulent flow from the wastegate and the External wastegate would be plumbed divorced from the exhaust.
Then you would remove the OEM exhaust valve from the muffler and relocate it, making it fully functional.
Meaning:
With the exhaust button off, the valve is plumbed in such way where the exhaust gas flow from the wastegate is merging back into the main exhaust, for instance downstream of the converters.
With the exhaust button on, then the wastegate exhaust flow is routed to a mini muffler, side exit, a la Porsche Turbo.

Functionality and sound:
With the exhaust button off, the external wastegate dumps downstream of the converters back into the main exhaust. Therefore the gases from the external wastegate don't experience the backflow caused by the restriction that the converters create. So the external wastegate that is always more responsive and has better holding capacity than the OEM ones, becomes even more effective. This allows the gate to stay shut for longer and increases the spool of the twinscroller even more.
Sound wise, it does become louder barely noticeable, when the wastegate dumps. a low rumble.

With the exhaust button on, now the external wastegate dumps into the atmosphere. So now, you really max out the spooling to where this supposedly larger size turbo with no internal wastegate (they spool better without the internal turbulence of gasses having to take all those turns caused by the castings of the IWG), spools like the OEM one, but, you also have a reduced backflow to the main exhaust gasses, with no added sound penalty, until the EWG dumps.
When the EWG dumps into the atmosphere, you have to get used to that sound. Its a 1-2 seconds LOUD sound. some like it. Pure World Rally Championship style soundtrack.




And the bottom is where the valve would be piped in.

Now that exhaust button in the dashboard has ESSENTIAL functionality.

All this from the Subaru world, but Porsche Turbo too.

In the Subaru world, it was possible to build a "divorced" wastegate and implement the above on a stock turbo too. This is because the IHI turbos have the exhaust flow from the turbine nozzle on one side of the turbine bellhousing and then the exhaust flow of the wastegate on the opposite side. So then a downpipe can be built that has an internal divider that protrudes into the turbo and separates the flows.
Have a look



Then a divorced pipe



And at the bottom is where you would relocate the muffler valve and have it work as a flow diverter, either sending the divorced flow back in, or to an external muffler.

Unfortunately, the bellhousing of our turbines appears to have the flow somehow ...coaxial. I'm not sure how to split this flow with a divider welded into the mouth of the downpipe, as above.