Does anyone have any experience with ASR CB2 exhaust valve controls ?

Not personally but a friend has fitted one to a 2023 s3. Takes a while to fit and need to get underneath the car and remove the undertray to fit. He is pleased with the result.  Not  massively louder but noticeable when  flaps are open. Negates the software that was changed when particulate filter was introduced that reduces flap opening so louder when wanted at the push of a button. Not sure how it affects warranty though.

Thanks for that. Not what I expected as I believed it was a fairly simple plug in behind a panel between drivers door pillar and accelerator. Warranty may be tricky unless easily removable without trace. I don’t mind that if it is straightforward as with long life service it is a decent gap. 
The other major attraction over earlier models is that the new facelift S3 has all this new supposed safety crap on it which you have to turn off each journey. This CB2 unit claims to permanently turn off start/stop, lane assist and audible speed warning. This is probably more attractive than exhaust noise !

Plug and play only refers to the electronics unfortunately. Disabling the safety bits is the bonus. Fortunately able to sort my 2023 car as can get past the component protection and access the controllers. Set my safety features to remember the last setting so still available if needed.