It does 400% increased by 50% per stack over 1.5s, while FoF does 2500% over 4s (upped by Crosswinds of 800 AP & 50% from Transfer of Power)
So FoF in theory will do 4,550% every 4s, or 1706.25% AP per stack over 1.5s.
This means you need a multiplier of 4.26 on SCK, that is ~ 7 stacks --- not feasible.
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The benefit of Serenity, I think, is that you can fit two Fists of Fury in one Serenity with 30% haste. While this can only happen in SEF once at the start (cause of two charges of SEF) but every subsequent SEF charge, you will only get one Fist within the time frame.
So in theory that will be 150% of FoF value for SEF or 2*100%*1.4 = 280% FoF value for Serenity. There is also the addendum that you'd net two FoF with 12s CD with Serenity (since you'll use the 2nd one within the Serenity window), so once you exit Serenity - you'll still get a faster FoF than you would with SEF (still incurring 24s CD)
In a five minute fight, that'd net 4 iterations of SeF FoF (3 + 1 since you get 2 charges at the start) or 3 iterations of Serenity FoF; in other words, 600% FoF or 840% FoF (and the difference is pretty giant).
The reduction in CD also affects strike of the windlord, again in theory netting you more applications of SotW in a fight than SEF would. You use SotW inside the Serenity frame, it incurs a 20s CD, that is every 1.5m you can cast two SotWs within the same window as you could cast SotW in one window (SEF).
So again, in a 5 minute window, you'd get 7.5 iterations of SotW for SEF v. 10.5 iterations of SotW with Serenity.
So we are practically comparing (150%)*[Increase multipliers]*[Combo Strikes value] value on all abilities used within the time frame of SEF versus
100%*[1.4+Increase Multipliers]*[Combo Strikes Value] in addition to casting more FoFs and SotWs for Serenity.
The big issue why Serenity will fall off (if it isn't already there) is due to the legendary adding .6s per Chi spent to SEF but not to Serenity, and the damage value of SEF scales much better since the 150% (50 each * 3) is baseline rather than Serenity having the 40% as an increase bonus (not even a more bonus