If you push the shadows down that is the same as lowering the ISO. So what you mean is that ISO 10,000 is too noisy and you push it down to 6400 to get acceptable results. This is why crushing the blacks works so well at ISO 12,800 on the GH2 in black and white. CMOS sensors have a signal to noise ratio per pixel and if your signal is high enough it masks the noise which is why well exposed bright areas of the image are cleaner even at ISO 12,800. Think of a raw image at the native ISO of 800, it will look noiseless over areas of a shot where the luminosity only requires ISO 200, and where the light is dim you need to adjust the curve to bring up the shadows and that is where you see noise.
The 50mm at F1.4 remark is a bit odd. There's a reason most films are shot stopped down to T5.6 as a default. You need a margin of error and for when something is moving back and forth quickly you certainly can't keep them perfectly in focus 100% of the time at F1.4, not even a ninja focus puller can do that on ALL types of movement. Some movement is random and you cannot account for it. With a locked down shot, of course F1.4 is fine. For most stuff, your actors will be going in and out of focus especially at closer focus distances, which is often not what you want.
I use very fast apertures a lot but only for locked down shots where there's no focus racking.