What is the utility of Onslaught and Upheaval?

[Chrono_Rising]

Didn't think it was that easy to calculate, I mean, you didn't account for the cd reduction of Infuriate from FC.

Yes, but their estimate is actually high in comparison to what it really is. This is because Fell Cleave is also on the GCD while Upheaval is not. So while the analysis that Upheaval is 450/20 = 22.5 potency per gauge, the same analysis for the average potency of Fell Cleave (590/50 = 11.8 potency/gauge) is not because you are replacing a GCD action. The calculation for the average combo potency is a bit harder to analyze on warrior than on dark knight since you have multiple combos giving different gauge amounts and also have an additional potency in terms of Inner Chaos reduction which is also dependent on the Average GCD potency. Basically you are correct, it is much more complicated than it was made out to be. Regardless, upheaval is definitely the most cost effective way to transform warrior gauge into damage.

Post Script Analysis
I calculated the average Fell Cleave potency: This does not mean every fell cleave is worth the same, but if you were to average all your Fell cleaves together this is how much potency it gained you over your average GCD. If you want more details as you what a specific Fell Cleave is worth you will need to consult your specific fight.
I did a quick calculation taking into account gauge lost, average GCD potency, timer reductions on Inner Chaos as well as Inner Chaos's interaction with its own timer. I did make a small approximation so this is an upper estimate on the average potency but I expect that it is fairly close to accurate.

For Fell Cleave: 1g is worth 7.15 potency on average. Meaning Fell Cleave is worth about 357.5 potency above your average GCD. Comparing this to Upheaval and Upheaval is approximately 3.15 times more effective at turning gauge into potency over Fell Cleave. Since they use different amounts of gauge Upheaval only gains you about 100 potency over Fell Cleave.

We can also use this to approximate how much of a potency loss onslaught is: onslaught loses 43 potency per use compared to saving the gauge to Fell Cleave. However, if you were to overcap your gauge with the next GCD, there by losing 10 guage you can onslaught instead.

That 10 gauge is worth 71.5 potency, onslaught loses 43 potency. That means using onslaught to gain back that 10 gauge is worth about 28.5 potency. In other words: Onslaught is a dps gain when used well. It isn't often a gain, but in the right situations it can be. This only happens rarely within your rotation. But also keep in mind that if your spring is down and/or onslaught would get you back faster and gain you GCDs it is a gain as your combo potency is also quite high.