The best theoretical possible noise level in a digital stream is typically defined by the amount of necessary dither. If a file is dithered appropriately for 16 bits, simply increasing the bit depth cannot magically reduce any noise (i.e. dither) already there.It is about less noise
In most cases the actual noise level in a real-world recording (typically from ventilation, traffic or indeed any analogue recording) far exceeds the level of noise required to dither a 16 bit stream.