cetane is how combustable diesel is when compressed, higher cetane means its easier to combust. therefore a higher cetane fuel will not give more power, it will simply make it easier for the engine to achieve combustion. unless engines are adapted to make use of diesel that is easier to combust there will be no benefit. you could argue that it will be possible to inject more diesel if it has a higher cetane rating because you might as well make use of all the compression the engine can achieve, and more diesel means more pressure in the cylinder when it combusts, but the limits of oxygen requirements cancel out any possible gains here, and also the fuel injection will not consider a higher cetane fuel is in use unless the ecu is remapped to compensate for it.
for all standard diesel cars the engine would still only be able to combust as much diesel as before, yes it is easier to do that, but the amount doesnt change. changing the cetane rating does not affect the pressure created in the cylinder, it only makes it easier to cause combustion. so its common sense that the power and torque figures should not change.
if we then remember that an engine's efficiency will impact the actual performance, and the efficiency of a diesel engine will be improved if the diesel is easier to combust, then we expect to see an improvement of performance. this is why a cetane rating of 40 would show a noticable loss of performance compared to a cetane 50 rated fuel. however once you go past cetane 50 you see no improvment, that is well know and proven. standard diesel is usually in the lower 40s, while the better stuff is in the higher 40s. if shell v-power is a 50 cetane fuel then it is the best you can get. but you also need to remember that the differecne in performance between a low 40s diesel and a 50 diesel wont be very noticable because the engine management is designed to use a low 40 cetane, and so the fueling is adjusted to match that level of combustibility. if you remap the ecu to be dependant on a higher cetane then you can unleash far more of the possible performance gain, but the difference between a standard diesel in the low 40s and a perfect diesel at 50 will not be huge no matter what car or setup you consider.