I used to have a set of aftermarket alloys fitted with winter tyres. My insurance company at the time was looking for a hike in price of 60% if I fitted them during winter months - despite the list price of one Alfa alloy being the same as the full set of 4 aftermarket ones! Needless to say I changed insurance companies.
I now use genuine Alfa alloys during winter months and I have never had a problem with insurance companies. As said previously, tyre treads and patterns and sizes (provided they are appropriate for the vehicle) are nothing to do with insurance unless they are illegal.
"When you sit in a Focus RS or a Golf R32 itís like sitting in a commercial for Lynx aftershave. When you sit in an 147 GTA itís like sitting in a Venetianís hand-made suitcase." J.Clarkson 2003.
Current fleet: 156 Sportwagon jtd 16V (facelift) 2.4 Spider, Leon Cupra SC
Projects: Spider S4, Matra Murena 2.2