Taking a look at something like this;
https://www.warcraftlogs.com/statist...unt&dataset=90
At first glance you say "Priest is the 2nd worst DPS class!". Which would be true here. However there appears to be quite a disparity between two groups, you have balance, demo, arms and enhancement together quite above everybody else.
'everybody else' is quite tightly tuned, the difference between top and bottom (fury to assassination in this case) is only 15%, and that's considering that not many players will be playing the weaker of their available specs. The difference between shadow and fury is only 6% which I'd consider quite reasonable.
I don't believe shadow is significantly underpowered, I do think some of those top specs should be reigned in mostly due to their AoE abilities.
I think shadow is suffering at the moment since it doesn't
excel at anything. We're not the best single target nor cleave nor multidotting. However we are lower than average AoE, we need something to counter that.
The 6.1 changes should address much of this, the lvl100 talent changes alone will do much to diversify shadows abilities.
Fingers crossed it doesn't take too long to reach live