IMO.....whatever spec you have the best weapons/gear and well optimized and reforged accordingly....except Sub that is, Sub. shines more and more as you gain better gear, that's how it scales, pre SoO not so good.
Regular standard advice around here, check with shadowcraft to optimize gear at any level first and foremost, and might check askmrrobot and icy veins for some extra info.
Assuming ToT gear, Assassination>=combat with 4pc T15 according to the type of fight, most ToT is more accustomed to single target though, so might stick to Assassination imo.
Regarding 545+ levels, they are so darn close, again assuming equal gear levels and appropriate gemming/reforging, 1-3% difference lots of the time.
High skill level and good at control? then sub will give you amazing numbers, except maybe if you had the kind of dancing tank I have to deal with so I don't even try anymore
Combat, with 4pc T16 and right trinkets, is pulling ahead if you can manage your cds well enough.
As for Assassination...same old really, and safest bet for most rogues, I would stick to it coming out of ToT, and switch to combat when you get decent weapons/4pc and trinkets.
Want to use 2 specs on same gear?, safest bet would be Combat/Sub, most of the time they have same gemming and reforging, while Assassination can go a couple of different ways, mastery/haste heavy depending on a few things like legendary gem and such....at full normal gear levels that differentiation kinds falls off and stat weights change fluidly and near equal.
I coined the lfr fist weapon today in lfr, and that puts me with two SoO lfr daggers and the combat weapon, at around 515 ilvl would assass still be the best spec?