Playing the PoJ build and, at my current asp, I reach 85 TR flurry stacks between the end of a cold cycle and the begining of the next one, WITHOUT StI.
I just did 125 and now trying to refine as much as possible to improve that score.
So my question basically is : "which one is better between StI and determination (is determination multiplicative ?) ?
I am using Azurewrath and WKL (1.85 sheet asp). Thx for the help ! : )
EDIT :
Well, just cleared 126 with determination so I guess I got my answer ! 
P.S. Had my flurry stack in the 90+ range most of the time.
StI is 30% damage multiplicative, because attack speed for some reason increases flurry explosion damage. The only drawback is that on rift guardians you will probably not have 100% uptime. Determination is additive damage and the same type with Momentum. Momentum will have more uptime (100% uptime everywhere) than Determination, so if you for some reason dont want to use StI, then swap Determination for Momentum, same exact damage but guaranteed uptime compared to Determination.
EDIT: By the way, attack speed doesnt increase the way you stack flurry, StI works like a 30% dmg multiplier to Flurry while you hit monsters with more than 70% hp, Relentless Assault as a 20% multiplier as long as monsters are frozen,blinded,stunned , Momentum is 20% additive damage as long as you move (you always do).
rentless assault is multipler.
use both and done. additional hsrmony and beacon
Thanks mate for this really detailed answer ! Reverted back to StI ! 
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