Ok, as ive enhanced my Impreza a little bit im quite clued up here (which makes a change )
Wheels...
I have Prodrive 17 inch alloys on my car, they cost me extra due to the fact they are not a standard part of the car (but they are a dealer accessory!)
Brakes...
Ive upgraded to a 305mm Brembo set up with AP discs, cost me nothing as it improved the cars ability to stop but some insurance companies see this as the opposite
Exhaust & Back box, ecu and intercooler piping (PPP) Dealer option, cost me approx 10% more on my insurance with 20% increase in power
Decat down pipe (sports one )
Cost me extra.
Air filter.
Havnt got one but looked into it before i found out they naff me MAF sensor up, wouldnt cost me anything as its consumable.
Fuel
oh well, consumable, the car drinks the stuff but the engine prefers the higher octane of Optimax due to the ECU being mapped for 97 ron fuel
Clear indicators.... dont have any but would have to declare even though they are a dealer accessory due to them changing the exterior of the car.
Side protection strips.... nope didnt need to declare these
Security...
They wernt bothered!
(the reason being is they know that these cars will be protected ) Ahhhhh i love LV
Spoilers... got the standard one on, if i uprated then yes i would have to declare it
Basically, consumable parts (sparkplugs, leads, oil, filter etc) which are within the recommended spec (ie dont add that much power) aint a problem, Brakes can be if you add different combinations that dont improve your car but do the opposite due to the fact they are not up to the job need to be declared (and its a safety issue along with suspension and strut braces as it alters the way the car handles).
Hope that helps
Tony