Simcraft's results will vary not only depending on your gear. They depend on the encounter model you select and the talents you use. Same goes for your actual stat weights. E.g.. Your stats have different weights for Iron Juggernaut than for Protectors.
There are no definite correct stat weights, and simcraft is not wrong except on rather rare occasions when certain interactions are not taken into account while modeling. The most common simcraft failures are pebcak errors.
The 1:1 crit/mastery ratio is a safe all-around reforging strategy. You can safely reforge +/- 2k secondary stats with simulated results varying less than 2%, which can't be significantly distinguished from RNG and player error.
There's a lot you did not take into account with your simcraft results. Including breakpoints above and below your current levels, secondary interaction with eachother and how much primary stat you have.
For example, if you were to run simcraft with your gear having 800 less haste but 800 more mastery or crit, the results would likely be reversed, as you would be directly below the DP and SWP +2 breakpoints. Which are very valuable in low gear. With that in mind, once you get above that breakpoint, haste is valued less as it effectively means you'll spend more time refreshing dots the more you get (its why being directly above a breakpoint is important right now, Your dots go back to full duration and you get that extra 1 second'ish to cast fillers every full dot duration.)
The important thing to gather from your sim is that where it currently stands, aiming for higher haste breakpoints is not likely worth it. Int still is valued higher than 2 of any secondary, which means you want to gem for red, If you're matching sockets, red will always be a component in your gemming. Brilliant in red sockets, Purified in blue, Reckless in yellow. Once you reach a slightly higher gear level, you'll see a point where your highest secondary passes the 0.5 mark. This is where gemming secondary will start to pull ahead. So it'll switch to reckless in red, quick in yellow, energized in blue. (keep in mind these are rough generalizations, exceptions do exist to this).
For most shadow-priests, this is around the 520 ilvl mark. Amp trinkets, UVLS, and some set bonus' may accelerate this.
The next important haste breakpoint for you is up at 14873. Haste's value will climb steadly as you reach this point until it peaks and drops off drastically afterwards. The same will happen towards the one at 50% (18300ish) However, once you are able to hit the 14873 breakpoint, mastery and crit will be so strong that its a very minor change in overall dps to aim for the 50% mark, especially true with t16 2pc equipped.