Devastate actually scales with gear. While sunder does not.
Lets assume 30% (5 sunders, 7700 armor base) mitigation, and we need to deal 201+ damage to beat sunder threat. Doesn't sound hard right? Well... We need a total of 162.5 DPS minus 'XYZ' DPS from weapon, with a 1.6 speed base. And with a 2.6 speed we require 100 DPS minus 'XYZ' DPS from weapon.
With gromtor's charge; 1.6 speed, 67.2 dps weapon. It takes 1334+ AP to make it deal on average sunder threat.
With say, king's defender, 87.5 dps 1.6 speed, it takes 1050+ AP to make it better than sunder.
The unbreakable will, 100.2 dps 1.6 speed, 872+ ap makes it better.
What this means is, with a higher DPS weapon, your AP scales better for threat with devastate.
If we switch to a 2.6 speed weapon (MH dps weapon)
the highwarlords cleaver, 71.6 dps and 2.6 speed, takes 397+ ap to make it better threat than sunder.
The merciless gladiators cleaver, 97.5 dps 2.6 speed, takes 35+ ap to make it better than sunder.
AND with
Syphon of the Nathrezim a 100.2 DPS 2.8 speed weapon, your (with 5 sunders) devastate will always deal more threat than sunder. You gain 400 AP from switching to this weapon from the merciless weapon for devastate purposes.