Indeed - as you say - at the end of the day, it's down to the insurers....alot of cars at 10yrs can be insured as classics (i've seen it advertised), but whether they'd see the 'R as a classic....that's the question.
This is why i've only ever thought about it, and not persued it.
I used to insure my 300zx Twin Turbo with a bunch of mods as a classic with the insurance factory http://www.insurancefactory.co.uk
It cost me around £350 with mods declared and not using any 'no claims bonus.'
I was so gutted when they said they couldn't negoatiate me a deal with my GTR and my premium went up 6xfold