I think there is a definate grey area here...
You don't need to inform your insurance company every time you replace parts on your car. If you happen to be upgrading them with "OEM compatible" parts then I don't see how they can legally decline you for having these specific parts installed on your car.
Now where I can see they would be able to have a case against you is if you're running around with your car 2mm from the ground. But at that point you're not installing a part to make your car better you're just doing it to look cool (I'd say good but that is far too subjective) and for most purposes less safe.
Installing coilover suspension and setting the car at a decent height isn't going to raise any red flags IMO.