Does Devastate generate more threat per rage point spent than Sunder Armor?
Depends on your spec and how much your Devastate hits for.
The scenario that favors Sunder Armor the most is when you have 3/3 Focused Rage and 3/3 Imp. Sunder Armor, meaning that Sunder costs 9 rage and Devastate costs 12 rage.
With Sunder generating a flat 301 threat for 9 rage, Devastate needs to generate 401 threat to compete. This means that Devastate needs to hit for 300 damage to become as rage-efficient as Sunder in threat alone. While hitting for 300 is certainly achievable with level 70 blue DPS gear, particularly with a slow weapon, in situations where you are tanking and your survivability matters most, 300 will probably remain out of reach for a while.
However #1: Regardless of whether or not you generate as much threat per rage as you do with Sunder with sub-300 Devastates, your Devastates are also dealing damage to the mob, when Sunder would deal zero damage. If the mechanics of a particular fight allow you to easily get ahead of your DPS classes on threat, Devastate becomes a better choice than Sunder.
However #2: While on paper the stat requirements for Devastate to compete with Sunder on threat-per-rage alone may seem like an issue, it isn't always the case in-game. Many situations where you'll be using Devastate to hold aggro are going to be boss fights where you should be generating plenty of rage to also spam Heroic Strike in-between global cooldown cycles. Shield Slam and Revenge, with Heroic Strike as rage allows, will remain your main sources of threat, meaning that you'll be using Devastate as a rage-dump for some added damage on the remaining global cooldown cycles.
In other words, as rage becomes more plentiful, looking at the threat-per-rage ratio of a single ability becomes less relevant.
The opposite is also true, and in cases where you are starved for rage you still have the option of using Sunder for that particular fight.
Keep in mind that if you don't have Imp. Sunder Armor, Devastate only needs to hit for 200 damage to be as rage-efficient as Sunder Armor for threat generation.