Dark Pit can use controllable arrows if he feels like it, and the difference between that custom and Pit's normal/controllable arrow custom is not that big since both are very precise, especially at long range, which is practically the only time you should use arrows.
Both Pit's and Dark Pit's arms have super armor, and Dark Pit's does 0.5% more. I have no idea where you got you're information. The arm you want to use is stage dependent most of the time, seeing as Dark Pit's sends you diagonally upwards and Pit's sends you upwards. However, Dark Pit's can force gimp situations since it sends opponents offstage, setting up for gimps with arrows or aerials. Pit's can KO earlier on platforms, and Dark Pit's can KO earlier near the edge of the stage.
The Ftilt is the only outright advantage Pit has (I really don't know why it has more knock back, or even why Dark Pit's arm does 0.5% more damage, considering everything else bar the arms and arrows are the same, even the Ftilt's damage output). The KO percents are- Tipper KO%s 163%, 134% Sour KO%s 223%, 184%
The difference is 30%, and FSmash KOs sooner anyway, plus the only time you KO is with the tipper. It might make Pit higher on the tier list by 1 spot, but both characters are so similar that the stage makes the most difference.