Thanks pandakraut for the answer.
So I ran a quick playthrough until Washington in BG yesterday/ today, and I found overall that the mod is very pleasant as it increases the quality of the game in many aspects (especially the changes in the career points and in the armory section, and also the strong importance of the fatigue management), but it seems that it accentuates (at least on the BG level) the fact that the game becomes relatively easy once the player has one or two corps with mostly 2 star brigades (which happens fairly fast, by 2nd Bull Run for me). One possible explanation for that is that, taking into account all the accuracy perks, the weapons are far deadlier than in the vanilla game.
To back that up, I have run a quick analysis (see attached), and the result is that if you fully specialize a unit in accuracy (3 perks which each gives a +100% plus the MG perk "Fire specialist), you end up with an accuracy factor of +400%, i.e., a 5x multiplier of the basis accuracy. Using the formula from pandakraut, it means that all the weapons in this mod are all far deadlier than the Lafayette (which has a max damage of 13.1 per shot) in the vanilla game. Note that I use mid-point of the accuracy values and not a range.
It's actually the same with artillery (even more if you include the damage bonus for level 1 perk "Long Range Focus"):
Overall impression what that after 2nd Bull Run, I was on the WW1 battlefield with armies that get slaughtered very fast (especially the IA as in BG it doesn't have much experienced troops) and by super precise weapons. And I confirm that the Whitworth guns are just brutal (killed more than 15k Federals at Antietam with them).
I am going through Legendary now, but there I have a hard time defeating 5k brigades at 1st Bull Run with my much smaller forces ...
@pandakraut: Does this make sense to you or am I wrong somewhere?