Oh, they have - but mostly they can't just decide to not kill humans or something - it's a basic instinct that most of them can't avoid : "Human ! Kill !"
..but it's like Arrancar doesn't like killing humans too
BTW Grand Fisher was one of Aizen first productions
Two words: concentrated power. Imagine for a second that all your energy from your natural form was now concentrated into a much smaller form. What would happen? Easy, a power-up, speed-up, heck even the healing abilities would go up unless you had some other way of doing it. I think all the sword does is stop you from using your most powerfull techs to increase your over all behind-kicking ability.
In addition, how people are talking an intelligance increase, I will refute that to a point. I don't think their base intelligence goes up, but rather that their capacity goes up (in short they still have a need to be taught).
With the "wall being torn down" all that that means is that now they can use the energy of both the "plusese" and the "minuses" verses only the "minuses" or Hollow power.
Shinigamis that turn into vizard gain powers up and hollow abilities you understimate the arrancar, what they gain?, DUH, that sword that they didn't have before is not fancy you know?, and its well know that hollow that become arrancar become more stronger so whats the diference between them, nothing that i see, just that vizards can do bankai, and we still dont know what more they are capable of, we saw grimmjow gran rey cero(royal hollow flash) that ony espada arrancar can do an so, arrancars are good, its just that they havent shown their big guns