Just a short post; this is how I think hold firm should work
+2 melee defence
Radius/area of effect similar to warcry
I think this way you more or less get what you pay for - it'll give you a small localised rally-like ability that will be helpful in shoring up a wavering line, but not so strong that it can tip a melee combat by itself.
I also think that Banzai should drop to 70 koku to reflect the fact that it is nowhere near as useful as hold firm.