To wrap things up, we basically want to go lighter for two reasons: To reduce the rotational inertia of the wheel + tyre combo and to reduce the unsprung mass. The former is not a really big deal, in my opinion. For fuel consumption, in constant - speed cruise, rotational inertia does not have any effect because the car is not accelerating. In variable speed driving, yes, the car would accelerate easier but also decelerate easier when you coast the car, two effects to cancel each other, in terms of fuel consumption. The main effect of reducing the rotational inertia is related to performance. In this respect, I don't really think that, it will matter for a car that is that powerful. If we were talking about a supermini with a 1.0 liter naturally aspirated engine, even a small reduction in rotational inertia would definitely be felt, but, this is hardly the case for a 2.0 TSI vRs.
But the latter, unsprung mass...I think this is a completely different story and I guess my positive experience about downsizing the wheels is mainly related to this.