Frankly, I don't see how you could call Denmark underpowered. Since it seems to survive relatively intact in most of my games it seems to be performing pretty accurately to me. Venice, too. Brandenburg is more of a borderline case but how much of their historical result was due to luck/mistakes of their foes?
4 of your remaining 5 under-performers are a result of the combination of the GH not behaving in a historic fashion (internal squabbles, collapse, etc) and the ai not having a clue on how to fight defensively. Add in issues with the war-dec and peace ais and you get the current "norm" of anything but green in OE land, no Russian unification (unless being conquered by Sweden counts), and a multi-hundred year early partitioning of the P-L Commonwealth. These seem to happen in 90% or more of games and pretty clearly point out that some things need to change.
France is different in that it either succeeds (and then some) or collapses spectacularly with no middle ground or coming back from the collapse. I'm not sure what the distribution is for others but I'd say probably 60/40 in my experience absent player intervention. I could see an argument for making France a little stronger than that. The bigger problem I see with France and, to a lesser extent, England/GB is the ease and frequency of collapsing once they have formed up. It is currently too easy and too likely that France, once formed, will spectacularly collapse due to losing a single war.
