I've told my insurer about induction kit, spacers, exhaust, rear lights, ... but not brake pads, stubby aerial, tyres etc!
Although they dont charge extra for these things, £25 to change the policy document each time is a bit steep though in my opinion!
matth76: I wouldn't mention the panel filter personally.... unless you plan to do a more obvious mod later, then declare the two at the same time.
would an insurer think it reasonable that an average motorist would take his car apart to find if a non-standard filter was hidden in a standard airbox?