>>3760914retard, it depends on the camera. There is amplification of the light collected at two stages after collection, analog amplification and digital amplification. If your camera does the same level of analog amplification at all ISO settings, congratulations your camera is ISO invariant, If however your camera changes the analog amplification the raw files will be different and you can not get the same image just by adjusting the exposure slider in post.
All of that is irrelevant though because the noise floor is constant and underexposing, no matter what ISO your camera is set at, will create a noisy image.
