Hey there guys!
At the beginning of 5.4, I felt uncertain about an unholy -> dw switch. Not only I had two weak weapons (2 x 530), I also had to swap some crit+haste H gear with ToT N mastery gear, losing a couple of ilvls in the process.
The lack of concrete DW info from other raiders, a disappointing initial impression of its DPS (extremely sensitive to downtime and parry/dodge) and absolute lack of mastery plate drops in our raids (horrible luck with warforged seals too), led me to almost go back to unholy. However, I decided to stay DW and wait until we were further into the patch.
Here's an "enhanced" raidbots graph of my Malkorok 25N logs. Aside from having void-zone soaking downtime/front stacking (parry)/displaced energy downtime, it's practically a patchwerk-type encounter, and I feel confident enough of my performance to draw conclusions from this information.
Frost DW Malkorok 25N parses:
I know what you are thinking - yes, the gear curve is that huge. You may even think that this could be related to human error, but I can assure you that all these results are well within 5% of simmed DPS, even ignoring downtime and other mechanics (i.e.: last kill dps: 280k/simmed patchwerk dps: 285k/simmed light movement dps: 277k).
So, what conclusions can we draw?
Clearly, lower than avg ilvl weapons are a no-go for DW. If you've had bad luck with your drops, stay unholy. I wouldn't go for 2H frost with a bad weapon either.
2pc is a must. Period. 4pc offers almost the same benefit, and I'm sure that the CD-reduction trinket makes it scale beautifully.
About older tier sets, I didn't make the math, but I think you should keep T15 bonuses if you aren't upgrading with something that's like +20 ilvls higher (compare 2nd and 3rd kill - +4ilvls, horrible scaling).
Can't comment on trinkets. That's my weakest point right now, since I've had really, really bad luck with drops so far. I'm expecting a huge dps increase with the amp trinket, comparable to the weapon upgrades.
I hope this serves as a reference for those who just made the switch and/or are still wondering about DW, like I was a couple of weeks ago. Unfortunately, I don't think I'll be able to show you how the build keeps scaling with trinkets and further updates, since we are going for H Malkorok this week. Maybe I'll try to keep it updated with H Iron Juggernaut, but we'll see.