A good thing to be aware of is that octane is a measure of the fuels stability under compression or its ability to resist 'knocking' and techically not its power -although the higher octane rating allows the power to be extracted more efficiently and is thus a reasonable rule of thumb.
The power locked in a litre of fuel is the amount of heat it releases when it is burnt (measured in kilojoules, sometimes calories, or in British Thermal Units (BTU)). The more heat you have per gram of fuel, the hotter you can heat the air in each cylinder, the higher the pressure in each cylinder, more torque at the crank, and ulitimately the power you drive though the tire.
The point of power being heat is best illustrated if you compare petrol to alcohol.
Petrol gram for gram contains much more heat than alcohol -which you would intuitively know if you've lit a little of each with a match
. Compared on this basis alcohol is a pretty poor fuel with a heat output of 8100 BTU/lb vs 19,000 BTU/lb for petrol. However alcohol resists high compression much better than petrol, giving it effectively a much higher 'octane'. Alcohol fueled engines run compression ratios typically of 15.5:1. This one of the key reasons why you can get more power out of an alcohol fueled engine, the other is that alcohol actually contains oxygen, so the fuel air mix stoichimetrically is around 6.5:1 or methyl alcohol and 9:1 for ethyl alcohol, vs petrols 12-14.7:1. So you are burning a greater amount of poor fuel (around twice as much) and harness more of that heat produced via the higher CR.
Petrol (unlike alcohol) is a mix of a wide variety of hydrocarbons molecules each of which offers a variety of heat outputs per weight when burnt. This proportion of these hydrocarbons in the blend varies quite a bit batch to batch out of the refinery, and also varies depending on which refinery it manufactured, and what base oils it is produced from. Additive packages are added to the resulting refined fuel to, among other things, bring the octane rating of the fuel to the advertised level. The result is that actual heat (and thus power) contained within each batch of V-Power or Ultimate (or Supermaket Brand 95) can vary. BTW the British Standard BS EN 228:2008 doesn't cover a fuels heat content.
Its also quite fesable to have a 'good' 95 Octane fuel having more heat than a 'poor' 97 Octane (whether that would translate into more power is another thing)..... If this ever happens is another question... and don't expect an answer from the oil companies.
The amount of power in a tankful can vary from tank to tank and from serivice station to service station for the same brand/octane fuel. This variation can be of the order of a couple of horsepower, so it would hard to say whether on any given day whether V-Power is more powerful than Ultimate.
By the way, fuel purchased in winter is better value than fuel purchased in summer as you get more kgs of colder denser fuel litre and thus more heat.