Additional security on an "unremarkable" (in the insurance company's eyes) car like a Golf (even GTI/GTD) won't really reduce the risk writedown any further. Put that additional security on something with massive specialist thief appeal like an M5/Porsche 911/Audi R8 and then you'll see a difference. A neighbour of my cousin has a detached garage like a fortress for his M5 (electrically retractable bollards, very high end detection and alarming in the garage, tracking etc) and it makes a difference for him.
You've got to ask what your life is worth. Leave the car keys where a determined burglar will find them if they break in or have them ambush you in the bedroom armed to the teeth, demanding you tell them where your car keys are?
My house security is pretty good, we're in a quiet and safe area (for crime in general), but if someone bursts through my bedroom door at 3am and sticks a sawn-off shotgun in my face and demands my car keys, they can have the car, it's insured.