You guys should watch Fifth Gear. There's this one episode where they did a test using an identical car with 3 different petrol. Normal unleaded, Shell Optimax (I guess should be the same as V Power?) and BP Ultimate (most expensive 98 octane petrol). What they did was fill the car up with a specified amount of fuel and they dyno the car (yup, dynoed it). Result? For an ordinary family saloon, there isn't much difference at all amongst all the petrol. Then they did the same test using a high performance car and immediately the difference is more obvious, the more expensive petrol giving a marked improvement in horsepower and torque.
I'm not making this up, go and download this seasons Fifth Gear. Should be one of the latest episodes.