Oh please

.
What it says on the V5C has sweet bugger all to do with the insurance.
My car is still down as a 1.6 on the Logbook. At the end of the day I still pay exactly the same amount of road tax as I would with or without the V5C saying "1998CC", if the tax system changes to put my car in a Higher/Lower band as a 1.6 than it would with the 2.0 engine then I'd mention it, for right now it isn't doing any harm what so ever. The engine and other modifications are declared to my insurance company and that is what matters, and they couldn't care less about what is written on that document.
Try and have a clue about the topic you're ranting about next time.
Anyway, The "booked as a 2 litre" comment suggests to me that a bigger engine has been fitted?