Stats priorities are all a mess because Intellect, Mastery and Haste can all be interchangeable between fights to mix/max your dps. As far as single target goes with low amounts of movement or no movement I like to gravitate towards Int> Hit cap > Haste 4717 > Mastery. High movement Int > Hit cap > Haste > Mastery. Add in 4-5+ mobs to a fight and Mastery begins to overtake Int and Haste BUT this is where it gets confusing. If you are moving quite a bit then Haste and Int's value start to re-take over if you can't keep up 100% uptime on every mob, but if you aren't moving very much and keeping up every dot, on every mob, then mastery is very good. Also, if you are in a fight where there aren't constant adds going out, say like in Will of the Emperor and the add phase is temporary along with more uptime to single target the boss, then mastery's value drops again. Amber'Shaper Un'sok's seemed like a good fight to gem and reforge for full mastery but I found out that with damage modifiers like in his final phase, is just doesn't keep up with Int and Haste. I might be biased towards being Hit capped, but as far as everyone's gemming and reforging ideas for Int,Haste, and Mastery...no one has the end all be all choice because we actually have three stats where one can be better than the others depending on the fight.
As for me though I like to keep a happy medium. Int is always the safe choice, along with a good balance in haste and mastery. So far that's been the 4717 haste cap and the rest going into mastery. Once I get a higher ilvl I will go for for the next haste cap as long as I don't have more haste then I do mastery. Hit capped of course as well.