Anyone with the knowledge to steal a Mk4 Jazz would already know it has a Honda tracker , but wouldnt know if its currently activated. I cant see many insurers giving a discount for duplicating what it already has.
And however expertly a third party Thatcham system its fitted there is an increased chance of electronic gremlins.
If the car is stolen to strip for spares they will probably have it stripped at a remote location thats not traceable to them before you even know the car has gone. I,m not sure i'd want whats left of that car back anyway.
I'd save your money. Make sure your insurance is 100% accurate. (I have a garage ,but its cluttered and in reality the car is normally on the driveway overnight - so thats how I insure it. )
And take precautions to prevent it from being stolen by someone electronically cloning your keys. Either habitually disable the keys manually every time you park , or keep them shielded in a Faraday bag.