Regarding Rend, I am still debating the usefulness of this since it's primarily based on Weapon Damage (IAS has no effect here). In response to IK 5 Set + Unforgiving, I am fairly certain that the 2 effects stack although IK 5 Set requires engagement with mobs before coming into effect.
Actually, I just bought missing IK pieces for the lowest price to figure out the bonus.
Fury DEgeneration starts 10 seconds after your last hit, be it breakable object or a monster. The rate is -1 fury each 30 frames so at 60fps you lose 2 fury per second.
IK5 set bonus is the exact opposite of fury degeneration, so +1 fury each 30 frames, you gain 2 fury per second.
These two effects apply at the same time in real time. When you hit an object, degeneration is canceled and you get a steady +2 fury per second from the set bonus.
Here´s the more interesting stuff. IK5 set bonus gets boosted by both Templar´s Inspire skill (+8%) and Animosity (+20%), so you get up to 2.56 fury per second from the bonus.
Also note that the game uses at least decimal numbers for fury globe. I just tested it and hitting ashes with Instigation and Mighty Weapons master with Inspire generated 16-16-16-16-17=81 fury in five hits, which means 15*1.08 = 16.2 fury per hit.
Unforgiving is slightly different that IK bonus because it straight up cancels degeneration just like when you hit something and gives you +2 fury per second (1 fury each 30 frames). Not only that, but Unforgiving ALSO gets boosted by both Animosity and Inspire.
Long story short, IK5 set bonus + Unforgiving + Animosity + Inspire generates 5.12 fury per second, which is pretty sick. The exact game mechanics are a bit confusing, but let me just say that the fury gen tick frequency gets boosted (frame length of the +1 fury tick gets shorter), it´s around 12 frames per +1 fury at the above set up.
Even more packed version: Fury degen -2 per second.
IK5 +2 per second.
Hit object degen canceled for 10 seconds.
Unforgiving +2 per second and cancels degen.
IK5 + Unforgiving = +4 per second no matter the circumstances.
Animosity and Templar Inspire boost both IK and Unforgiving fury generation.
For people using IK5 set with a Mighty Weapon to Smash, it could be more beneficial to use Animosity (+4% Smash crit +20% fury gen) over Weapons Master (3fury proc). It´s possible to calculate fury gen per second for each skillset.
Example 100% crit Smash
ITF = 0.8 * 15 = 12
Animosity = 12 * 1.2 = 14.4
Mighty Weapons Master = 12 + (3*0.8) = 14.4
90%/94% crit Smash
ITF = 10.8 / 11.28
Animosity = 11.28 * 1.2 = 13.536
MWM = 10.8 + (3*0.8) = 13.2
Same can be calculated for WW and RLTW (fury per tick or fury per second) or any other skill pretty much.