mate... u should be telling them where to go if they think u should pay 16v prices for an 8v car!!!
IIRC the 16v is 2 insurance groups higher than the 8v, it sits in the same group as the vr6!!!
following on from what golfpro said: you might wanna make sure the engine numbers on the v5 and the actual engine match, at least that would tell u if the car has the wrong engine in it. but as he said... it is very unlikely that it has had a less powerfull engine put in.