wow they get terrifying in a good way when they are so fast. Immediately makes me target the torso or knees just to stop them from running :). Also that head popping off clean like a bird head is just magic.
Very difficult question, I think you should try to get away with the least detail possible, because when you start adding all the organs and stuff it’s really easy to notice when things look “off”. For example, when you shoot the torso with a handgun and expect a few bleeding holes you get a hole the size of a cannonball, but all the ribs are still intact. The more detail the more gore visuals would have to differ for various impact types which adds a ton of complexity. I think you can hide a lot with clothes and by just swapping textures in less detailed areas. If you do wanna go all the way, with a hole that size all organs just drop out of the zombie 
.