On a one-minute single-boss fight, each 1 % of spell crit is worth 12.8 Spell damage, each 1 % of spell hit 12.8, and one Intellect 0.26. Shadow damage counts exactly as much as spell damage, since every spell you cast is Shadow. The weights use Spell damage as their unit and start from the benchmark set: they tell you what the next few points of a stat are worth from there.
Per point
- Shadow damage1.00
- Spell damage1.00
- Intellect0.26
Per 1 %
- Spell crit12.8
- Spell hit12.8
- Casting speed−4.29
Hit
Against a level-63 boss, 17 in 100 of your spells miss before you add any hit, and at least 1 in 100 always will, so hit is capped at 16 %. 
Shadow Focus gives 5 % to Shadow spells. The pre-raid set adds 2.0 %, which leaves your Shadow spells missing about 10 % of the time, far below the cap, so every point counts in full. The 17 % and the 16 % come from Classic's table, since Blizzard hasn't published Forever's. In Forever any item's hit counts for your spells, melee hit included.
Crit
In 
Shadowform, Mind Blast, Mind Flay, Shadow Word: Pain and Devouring Plague crit for double damage instead of one and a half, so a crit's bonus is double what a Classic Shadow priest got. Shadow Word: Death still crits for one and a half. Dot and channel ticks can crit as well. A spell's crit roll is separate from its hit roll, so crit has no cap. This set shows 18.2 %.
Intellect and mana
Every 59.5 Intellect is another 1 % crit, which makes up most of its weight. The mana it adds you rarely need. Spirit and mana per 5 seconds aren't in the table: with Shadowform's half-price spells, you don't run out on these fights.
Casting speed
Haste only shortens Mind Blast's cast and your global cooldowns. Mind Flay's ticks stay a second apart, and the rotation waits on them, so the measured weight swings between fights: -4.29 over one minute, 7.76 over two. Treat it as unknown rather than as a stat to chase.
On every fight
Against one boss, hit and crit are close to level on every length. Against three, hit falls behind crit, to 9.72 against 12.8 on three minutes. Shadow Word: Pain is half your damage there, and a missed one is recast at the next global cooldown, so a miss costs one cast's time rather than the dot.