Improved Expose Armor is a 10 point Rogue talent located in the 3rd tier of the Assassination Tree.
Rank table
Rank
Armor reduction increase
1
25%
2
50%
Expose Armor And Maxing DPS
Expose armor grants 3075 Armor Penetration while 5 stacks of Sunder Armor only grants 2600 Armor Penetration. For this reason, Expose Armor is a better debuffer than Sunder Armor in terms of raid wide damage gains. The only caveats are that Protection Warrior's may lose a slight amount of damage from their Devastate ability as it gains additional damage from each stack of Sunder Armor and that 1 rogue must work this ability into their rotation.
External links
Raidguru Armor Penetration and Boss Armor Value Guide