I took a photo of the waxing gibbous moon at ISO 6400 and ISO 800. At first glance, I cannot tell the two apart. Usually, a photo at ISO 6400 starts to break down with splotchy coloring. This was not the case.
Of course, ISO 800 will yield a darker image than ISO 6400. Previously, I was adjusting the brightness, contrast, curves, highlights and shadows to reach the optimal exposure. The other way around the low exposure problem is stacking the same image in Photoshop.
Nikon D800, ISO 6400, 1/1,600s
Nikon D800, ISO 800, 1/2,000s