i actually find this to be somewhat false, shadow priests can be quite a bit under the hit cap if it means picking up more haste/crit/mastery/int (crit or mastery depending on how you gear, and set a percentage that you deem is too low for your hit to be) i keep around 16% hit on my shadow priest and rarely miss, although when i dip down to 15% or so i start to notice the increase in the amount of misses you get. I suggest keeping your hit around 16% and so long as you're somewhere in the ballpark of that number you should be fine. I also suggest using
www.askmrrobot.com for your reforging, you'll have to adjust the stat weights information to set the hard cap for spell hit to 16% if you decide to do this, and adjusting the weights for mastery and crit if you favor crit over mastery, as it has mastery set above crit by default