Just so this is all in one place, here are the BiS lists for a defensive setup and DPS setup both with and without 725 dungeon gear.
BiS for a defensive Crit>Mastery build (I'm not including 4-piece for this. If you need/want 4-piece you'll have to equip tier helm/shoulders/legs/gloves:
Head: Faceguard of Iron Wrath
Neck: World Ender's Gorget
Shoulders: Doomcrier's Shoulderplates
Cloak: Void Lord's Wizened Cloak
Chest: Stamped Felsteel Chestplate
Bracers: Wristplate of the Wretched
Gloves: Crimson Throatgrabbers
Waist: Annihilan's Waistplate
Legs: Legplates of Iron Wrath
Boots: Cruel Hope Crushers
Ring: Mannoroth's Calcified Eye
Weapon: Felstricken Sha'tari Crystalsword
Shield: Smoldercore Bulwark
With 725 dungeon gear (differences in bold):
Head: Faceguard of Iron Wrath
Neck: World Ender's Gorget
Shoulders: Doomcrier's Shoulderplates
Cloak: Void Lord's Wizened Cloak
Chest: Gutcrusher Chestplate
Bracers: Wristplate of the Wretched
Gloves: Crimson Throatgrabbers
Waist: Annihilan's Waistplate
Legs: Legplates of Iron Wrath
Boots: Cruel Hope Crushers
Ring: Mannoroth's Calcified Eye
Weapon: Felstricken Sha'tari Crystalsword
Shield: Draconian Doomshield
Getting Warforged pieces from HFC is far more likely and will be better than almost all of the Mythic dungeon gear. Taking Warforged into account, only the Draconian Doomshield from the above list is actually better than the HFC gear from the original list. So the only dungeon worth running every week is UBRS for the shield.
BiS list for a pure DPS build:
Head: Casque of Foul Concentration
Neck: World Ender's Gorget
Shoulders: Doomcrier's Shoulderplates
Cloak: Void Lord's Wizened Cloak
Chest: Breastplate of Iron Wrath
Wrists: Wristplate of the Wretched (better than Breach-Scarred Wristplates because the haste on those is wasted as you're already hitting 12.5%)
Gloves: Gauntlets of Derision
Waist: Annihilan's Waistplate
Legs: Legguards of Grievous Consonances
Feet: Treads of the Defiler
Ring: Zakuun's Signet of Command
Weapon: Fiendsbreath Warmace
Shield: Fallen Defender of Argus
Trinket 1: Empty Drinking Horn
Trinket 2: Tablet of Turnbuckle Teamwork/Unending Hunger
With 725 dungeon gear (differences in bold):
Head: Goldsteel Greathelm
Neck: World Ender's Gorget
Shoulders: Doomcrier's Shoulderplates
Cloak: Void Lord's Wizened Cloak
Chest: Breastplate of Iron Wrath
Wrists: Wristplate of the Wretched
Gloves: Gauntlets of Derision
Waist: Annihilan's Waistplate
Legs: Legguards of Grievous Consonances
Feet: Treads of the Defiler
Ring: Zakuun's Signet of Command
Weapon: Fiendsbreath Warmace
Shield: Fallen Defender of Argus
Trinket 1: Empty Drinking Horn
Trinket 2: Tablet of Turnbuckle Teamwork/Unending Hunger
So yea, just the helm, and it's a difference of about ~30 DPS... Of course, getting a Warforged Casque of Foul Concentration is much more likely and is going to be better.
For anyone that's interested, here's the DPS, TMI, and DTPS differences between the two builds: