I was insured through Roadsure with my SJ and they took all mods. Unfortunately they will not insure vehicles under 5 years old so they would not insure the Jimny,
I am currently with Adrian Flux for the Jimny, with all mods declared - there did not appear to be a premium for the mods - just a pedantic document to complete to list the mods - and you need to give the name of the "qualified" installer for the significant mods.
Martin