We know very little of what the new passives for specs actually do, other than what the tooltip implies, which is "You gain 5% more of the X stat from all sources." So the assumption is it's not a modifier to your baseline stats, but rather the stats that come off your gear.
Now immediately this raises a concern for me. Let's look at assassination and combat.
Assassination gains 5% more mastery, and Combat gains 5% more haste.
Assuming (there's a lot of assuming here), that stats remain weighted in a similar manner as they are on live, I'm gonna make up some numbers for the sake of example (I'm far from being a theorycrafter).
Say Assassination has a stat weight of 2 on Mastery and 1.5 on Haste.
And Combat likewise has a stat weight of 2 on Haste and 1.5 on Mastery.
Now imagine that you're a combat rogue who's had some unlucky RNG and wound up with a lot of mastery gear in the first tier. Say you've got it at a ratio of 4 mastery to every 3 haste.
Your actual stat value, then looks something like this(excuse the numbers again, merely for example)
Haste (3*2)+(3*2)(0.05) = 6.3
Mastery (4*1.5) = 6
Total stat value of 12.3
Now if you're looking to get the most out of your gear, since you have no control over your stats anymore via reforge, the solution to the problem of bad RNG on gear drops is to switch to assassination, because your stat values then looks something like this.
Haste (3*1.5) = 4.5
Mastery (4*2)+(4*2)(0.05) = 8.4
Total stat value of 12.9
Now numbers are purely representative, again I'm not a theorycrafter. The end result is, that the only way to combat unlucky RNG on gear, and the only thing you truly have control over after Reforge is gone, is to swap to the spec your gear dictates.
Obviously I'm only talking about rogues, and to a lesser degree all pure dps specs, but it really seems like a design flaw to me. I won't be forced to play a spec I don't like to keep up with progression, and to feel like I'm not gimping my raid group.
Feel free to call me out for being a idiot on the maths, because alas, I am an idiot on the maths.