Personally taking your signature as an example I would only declare the following and I would state them exactly as below:
Intercooler
Stainless Exhaust System
Lowered Suspension
Induction Kit
And that small list of 4 items I got from :
Ming Blue S3, Custom Stage 2 Remap, Forge FMIC, 3" Downpipe, 200 Cell Sport CAT, Full Stainless exhaust, Full Red silicone hoses, Forge 007p, KW Suspension Kit, Green filter, Custom tip, 11mm & 16mm Forge spacers, H&R Anti-roll bars, RS6 style 18" alloys, Michelin Pilot Sport 3's, Red Dog Bone Mount, Neuspeed quick shift, Goodridge brake hoses, Aero wipers, LCR splitter, Forge Tie-bars, LED internal/external bulbs, Armrest with cupholders & TT pedals.
Insurance company is not interested in stuff like the dump valve, wheel spacers, quick shift kit, dogbone mount etc etc
All they really care about is performance enhancing stuff and mainly the stuff thats actually visible to an inspector. Stuff like the remap should be declared legally, however you could argue that its not really able to be proven to have had a remap as n insurance assesor is not going to have the skill or knowledge to pull the map off the ecu and compare it with a stock one. So its upto you really.
Personally in the past I have delcared all the big stuff. On my 640bhp skyline they were advised of the aftermarket ecu, exhaust, wheels, suspension, bigger turbo, intercooler, bodykit and the power increase as a % and with that they were completely happy. I didnt go into detail about twin z32 MAF's, rods, pistons, etc as quite frankly it would mean nothing to most of them !
On my MX5 when i turbocharged it I simpy declared a turbo kit and a 30% power increase. That covered me for the turbo, the ecu, the induction kit etc etc and when the car was involved in a smash they inspected it and paid out without any hassle.