I would be inclined not to state any figures to them, merely to state that it is Land Rover part from another vehicle in the range. Surely its up to them to calculate the risks based on facts not guess work.
When I insure my racer I list all the modifiactions (or at least all that I can remember), but don't try and figure out what benefit fibreglass doors or uprated brakes would have.
As Porny states without fuelling changes the performance benefit will be minimal anyway.