I got the iron jug belt normal and I used to have the AH belt, after gemming and reforging it i gained like 3% mastery and 1 % crit and i was expecting like at least a 1k increase in dps but simc is showing a loss of 300 dps.
Same Ilvl, different stat weights give different DPS based on specializations.
This only tells that, in your case, the crafted belt (which gives haste) had given you a breakpoint or something that gave better dps (but keep in mind that sim is just a simulation, sometimes it's very true, sometimes it's not, and you need to know what you're looking for).
The number in the middle is just an average, gaining crit usually means your average goes down because crit is an RNG stat.
All 3 of the stats are insanely close to each other for the pure single target sim so it really wont make much of a difference. In raid especially during cleave / aoe fights you should see a small dps bump.
depends a lot on the fight too. For instance my haste is very low, and on thock i have a lot of lower then usual dps because as his screams get faster it eventually gets to the point that the only thing i can cast are instant spells. Still killed him #1 on the meter because thats not the entire fight, but for single target int/hit cap > mastery > crit > haste
I had the same problem, so i am deciding to go Mastrey>haste>crit reforges on galakras, thok and spoils.
But i am thinking of going gosup fel imp on thok ??? or going gosac and summoning the imp when the boss is at high stacks of shouts ?
AoE dispel from your sup imp on thok when the raid is clumped up can be a huge help....its not always about the pew pew numbers. Also the casting restrictions during thok mean a pet will do more even when you sim/normally do more as gosac destro. Simcraft models a very sterile bubble of a world which shares only spell mechanics with the actual raid one. Understand how its data is useful but don't take it as gospel....especially when adapting to unconventional boss fights.
Your minimum and maximum should never be something to aim for anyways. You should always aim for a higher average dps.
Also, have you considered using exp/mastery instead of int/mastery? This would let you reforge away even more hit and then you'd end up with even more secondary stats. I do this as a fire mage since all my 3 secondary stats are relatively close in value and they're all better than int when it comes to the 2:1 ratio. I'm not sure you're at the right ilvl for it yet, but it should definitely be worth it when you get above 560 ilvl, as long as you can reforge enough hit away.
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Also, don't trust sims blindly. I did two exactly the same tests with 50k+50k iterations and your "max" dps was 364k on one test, 372k on the other.