No worries. As it's an optional extra I would insure it as a modification as insurance companies only recognise them as standard models, you have to add the extra bits on yourself. It shouldn't cost much more to be honest, when I had my first 350z which I sold last year, it had the FF nismo kit, I was paying around £650, aged 25 and with 8 years protected no claims.
I know what you mean about the road tax, I'm currently looking for another zed with nismo kit etc, for the sake of an extra £200 a year I would rather have the face lift as I prefer the interior and the light upgrades. Personal choice though.