I'm not sure if there is an exact cap...but I think its moreso about efficiency vs losing all of that haste. To get the massive STP needed for a potential 4 hit, you'd have to TP in askar/usukane body with hachi gloves/hachi+1 with the STP+ keiton+1, with goading belt and AF3+1/+2 legs and head (which i'd main anyways in a haste build) usu feet, maybe some store TP+ ring (there is one with like str-4 dex-4 stp+4)
so to recap lots of STP+ but the amount of haste you lose is as follows:
feet: 1%
hands: 3-4%
legs: 1% (given the boost to hasso isn't haste...idk yet..i think these will have a haste boost, but i think its unconfirmed atm)
belt: 1-2%
also losing 2% DA on grip as you'd more than likely need Rose if the 4 hit is even possible.
so thats 6~8% haste lost for a potential 4 hit build, when now you can more than likely 5 hit the WS+10% dmg GK