Did you see the animal stunned in the first part of video one? It was out in less time than it probably takes to registrar it was even stunned. Animals can be knocked unconscious quicker than they can consciously process the information that they've just be painfully stunned. Pain, again being a precept, take time to be "felt". Perhaps more time than it takes to shut down the brain.
I believe those studies could be and should be done.
I've been knocked out, while I'm sure the blow to my head was severe and painful (car accident) - I didn't "feel" it because I was knocked out so quickly. Later, when I rubbed the top of my head, yeah, pain.
Also, animals react to the smell of blood and also to being confined. If an animal is sticking it's head through a hole for daily feed, I don't think it'd even know what hit it. Standing knee deep in blood, well, I'm sure the animal is pretty stressed.