This is not a problematic skill at all, because shortbow is a weapon set that is designed for utility and mobility is not only a major part of utility, it is also a defining feature of thief. When you think of weapon skill balance, you shouldn't look at weapon sets of other classes to decide whether it's broken or not. Instead you should look at the role of this particular weapon within the class, eg sword for sustained fights and pistol offhand for controlling fights by blind, stealth and interupts. Mobility alone doesn't win pvp games. Good thief players read the map and go to the right place at the right time with the help of shortbow and shadowstep, whereas bad players die in 1v1 and teamfights they are not supposed to be in. If you think the utility of shortbow is too much to justify having such a strong shadowstep skill, maybe you should look at the other shortbow skill and tune them down because I honestly think shortbow 5 is a defining skill of this weapon and is what makes thief at least competitive in pvp.