If I was just changing tyres and not the alloys + the tyres were the same size as those they replaced I would not tell my insurer .... they are a different tread and compound but then Pirelli and Bridgestone are too - I am sure an insurer would find it impossible not to pay out ....
I am all in favour of telling insurers about modifications, etc but I think we can all go too far trying to be 'good' and 'open / honest' + this has, in some cases, given insurers the opportunity to take the p*ss. Changing alloys as well though and I would tell them ....
Just my view FWIW.