I had a similar conversation with my Ins co when I had a new Motad exhaust on a motorcycle. Initially the Ins co said it was unacceptable, but I asked the CSA (Customer Services Advisor) what car she had and if she’d had the exhaust changed?
She replied ‘Yes, I got it done at Kwikfit’
So I asked if she advised her Ins Co of the change…
Silence followed and then she agreed that the Motad was acceptable.
One point worth mentioning though - Motad state that their exhausts add no BHP or performance boost but just better made in stainless steel than the originals.