when i renewed my insurance i had already declared my exhaust and wanted to declare my hfc's, after explaining about 3 times over what high flow catalysts were i was told that they were part of the exhaust system and therefore no need to declare seperately, this was with admiral and although they do not cover mods like for like it was hard work trying to explain something quite basic i would imagine when i fit my mrev2 and try to explain that to them it will still be classed as induction kit using their logic, at no point have i been asked for estimated gains for each individual item or the make/manufacturer, i think with admiral the only time that question is asked is when you declare a remap
as for gt pack i personally don't think that needs to be declared your car is a 350z gt, again when i insured mine i'm pretty sure it came up with options like 350z coupe,350z gt coupe,350z gt roadster and so on