This is a great example of the differences between systems and reality. IMHO Coolness under fire is definitely an acquired skill. Some have an aptitude for it (high relevant stat/high starting skill), others don't but can be taught, or, if they live long enough can learn themselves.
However in terms of an rpg you run into the same issues that MRQ did with Resilience/Persistance, that meant that once you had acquired the skill it was no longer a challenge.
This is inconvenient for gaming but is quite realistic in terms of firefights. The more you do it, the better you get at it. I think it is best expressed in Band of Brothers, "As soon as you come to terms with the fact that you're already dead you can start doing your job" or somesuch...
There is the counter argument that as soldiers become more veteran they actually lose degree of recklessness required for certain activities - I read somewhere that the general thinking was that only a unit that hadn't seen combat could have taken Omaha beach because of the level of naivety required to charge machine guns...
I'd probably go with your ideas as far as a game's concerned, since it is, after all, a game...
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