It doesn't matter what potency a specific GCD is, because we can't just decide to spam that all day. All our non-gauge GCDs are tied to our combos (+tomahawk), and you can't just pick one of them. You have to compare that 340 to the average of a full "rotation" of our combos (since you have to apply eye every so often, but do path otherwise) + the value of the gauge gain.
However, now that I did the math, assuming Eye + 2x Path rotation, and basing he value of gauge on Onslaught (100pot per 20 gauge), it turns our average combo GCD is as high as 376.67 pot.
So spamming Onslaught on CD is actually a ~37pot per 10sec gain over using FC to dump gauge. That also means that if you will miss more than one GCD per 100sec by not saving Onslaught, you should have dumped with FC instead. No considering raid buff windows.
In a mechanic heavy fight, I think using FC is better.
It also means buffing FC by 40 pot would be a good way to buff WAR

Then you can always save Onslaught for movement. Again, not counting raid buff window minmaxing.