Yea, after finishing a game as America on emperor, i feel they're sorely lacking compared to other Civs. I still won easily enough, but I couldve done what I did with any other civ.
I think at one point during Civ5 development, barbarians were supposed to be more of a threat and the visibility bonus was supposed to help them out a lot in the early game. Barbarians have a maximum radius from their encampment that they can't go past, so extra visibility helps keep their spawns far away from your cities. That bonus, however, doesn't really pan out since barbarians are pinatas filled with XP and gold in Civ5. Infact, their ability is deterimental since you WANT barbarian camps to spawn close to you.
Minutemen are a joke of an UU. The B17's are really nice, but they don't play any factor whatsoever in helping the US achieve victory. As someone earlier said, games are "won" before the modern era.
I think giving them a +50% settler production bonus, along with +1 settler movement speed, on top of everything else, would help them out immensely in the early game(and early game advantages leads to a snowball effect which can be very powerful in the hands of a good player... see france, germany, and songhai as an example). They'd be a rapid expansion civ, just like they are in real life. The Liberty SP would then be better than the Tradition SP for America. As it should.