You are playing Black Powder, it's not a detailed simulation of a Napoleonic battle. Think of it more like a simulation of HG Wells and his chums playing out a Napoleonic battle. So what the reality was is less important than the story that is being conveyed. Highlanders are fearsome hairy barbarians who are tough in battle. That's the story, in the same way the French are an unruly mob and the Russians can't shoot. They come on in the same old fashion and you see them off in the same old fashion (even if such a thing wasn't the reality at all). Trying to find historical precedence and even statistically significant data for such a thing is a fools errand.Probably the best post I've read about wargaming in years and can be applied to all rules and games.
If you want to be "realistic" you should get rid of the bulk of the National Characteristics, especially at Battalion level. Battalions varied as much within an army in performance as between armies. On their day many units performed well and on another day they could perform very badly. To me that is what the dice are for - is the unit on a good day or bad day? You roll the dice and find out.