Does replacing the stereo count as “modifications” for insurance purposes, or is that specifically handling and performance?
In a word “Yes”.
Whether they would or could invalidate your insurance having an accident is another matter of course, (if not informed).
I notified mine (Admiral) and it was logged as being changed. I think when I originally told them a few years ago they allow up to a certain amount (in money/worth) before they add anything to the premium for the change. Same with other additional mods, depending what it is and if it alters the BHP or not. Go faster stripes cost nothing.
I would as a matter of course just inform them no matter what mod you do …!
Hopefully you won’t but if you have any issues with insurance for fitting uprated audio then please feel free to drop me a line.