Is pistolwhipping considered deadly force?
Deadly force: that degree of force a reasonable and prudent person would consider capable of causing death or grave bodily harm (eg maiming or crippling injury).
Beating someone upside the head with a hefty piece of metal is likely to knock them unconscious, cause brain damage, or kill them. Any reasonable and prudent person knows that (as you do yourself; your post said so).
Therefore, yes. Pistolwhipping is deadly force.
Incidentally, it's a bad idea to use a loaded gun for beating someone. You could shoot your eye out.