This is the thing about Onslaught and its damage/cost:

When you use a Fell Cleave, you're not just getting a boost of 500 potency for 50 Gauge, because if you didn't use Fell Cleave, you would be using a standard combo GCD instead. In other words, the gain from using a Fell Cleave can only really be said to be the difference between using FC, and using the combo action you would have used otherwise.

Storm's Path and Storm's Eye combos each total up to 610 potency across three GCDs, for an average of 203.3 potency per GCD. Fell Cleave is therefore (on average) a gain of 296.7 potency over the combo action you would have used instead during that GCD.
However, while using Fell Cleave costs BG, your standard combos are where you actually gain BG. Assuming that you're rotating Eye-Path-Path, you're generating 80 BG for every 9 standard combo GCDs, or an average of 8.9 BG/GCD. Fell Cleave represents a loss of this 8.9 BG, in addition to its 50BG cost, so the effective cost is 58.9 BG.

Potency/BG for every Fell Cleave is then 296.7/58.9, or ~5.04 p/BG. And bear in mind that losing 50 BG in one go reduces your crit rate by 5%.


Now with that in mind, let's look at Onslaught. This one is actually really simple: When you use Onslaught, any gains and costs are as-is, because you use Onslaught in addition to the GCD action you would be using otherwise.

Onslaught is 100 potency, for 20 BG. You can do that without really thinking: 5 potency per point of BG. The 20 BG cost also only lowers your crit rate by 2% on your next GCD.


The similarities there should be pretty striking. Neither skill is an automatic 'use BG on this ASAP, every time' skill like Upheaval or pre-4.05 Inner Release, so both of them are your BG dumps. Both of them convert BG into damage at a rate of approximately 5 potency per point of BG. In a vacuum, Onslaught and Fell Cleave are approximately equally good for your overall DPS. Fell Cleave has the advantage of pushing your combo back by one GCD, which can help a lot with aligning your combos to your Berserk/Infuriate. Onslaught has the advantages of being an insanely good enmity tool, and giving you more mobility than any other tank.

Think of Onslaught as a way to do one third of a Fell Cleave, at exactly the same level of efficiency. Pretty much every aspect of the skill is powerful, and getting rid of the associated costs is essentially just a straight DPS buff to the tank that already has a DPS advantage - while robbing the class of a key gauge management skill that plays a huge part in how smoothly WAR can cope with any downtime or mechanics.