Yep mine covers up to a bag of sand too. Don't worry too much about telling the insurance companys about mods, when I tinted my old car it cost me an extra £12 a year. It's always best to check first before shelling out on kit though.
as said mate most is covered to around £750 - 1000 depending on cover
however if your going to be cutting the dash apart and installing screens and speakers in there place then expect to see an increase due to the modified interior