i didnt find it made much difference on my older 172's but i used it anyway, i wouldnt put cheap oil in my cars so i wasnt putting cheap petrol in, just part of running a performance car.
found the 200 doesnt run anywhere near as nice on standard 95ron as what it does on 98/99 ron. Found Tesco momentum is a good alternative to Vpower, good bit cheaper up here generally.
to answer your question on an NA car its more just smoother running, Mark cup got some interesting results with MPG when he worked it out, ive personally never paid much attention to my MPG and my driving style varies that much that its hard to notice differences