i doubt on a new car you would get that gain, but on an older car with worn wires yes, you'll see a "hp gain". in reality you'll just be regaining lost hp
maybe with some mods being done i could see a gain, especially if you had an ignition system installed, and maybe some specific applications where the factory wires just aren't good
but for our cars, most seem to say you won't notice a hp gain if you have good wires, but you will notice a little refinement in engine smoothness and throttle response
oh, and for our cars, everyone seems to like magnacores