+8 STR roughly translate to 4~5 base damage when you WSing, so it's roughly the same as Amano's base damage.
The special thing about Futsuno is its +8 STP on top of 480 delay allows you not to rely to any suboptimal STP pieces required for 5-hit build (Usukane sune-ate + Rajas + Sword itself already give you 5-hit build)
But the thing is Hagun gives 0.3125 higher fTP than normal GKT when you are WSing at 100% (and even more when you release it with TP closer to 200%). This makes WS damage from Hagun is roughly 20%~33% higher than normal GKT with similar base damage.
In merit PT, considering aggresive SAMs have about 35~40% damage from melee swing and the rest is from WS, Hagun got an advantage here. 1 reason why the damage distribution as such is due to meditate. And for /DRG, it's Jump, High Jump, and the DA proc from those 2 JA can be significant.
As for the calculation itself, I don't want really to bother since we're comparing a couple different aspect here. Probably the best one I can come with is : "Assuming the same amount of haste given, a perfect 7 cycle of TP->WS from futsuno mitama is the same as a perfect 6 cycle of TP->WS from 450 delay GKT"
EDIT : did some calculation a bit, there might be some error. Please recheck it:
6 Cycle of TP -> WS Hagun = 7 Cycle of TP->WS Futsuno
Assume : +8STR from Futsuno give fSTR+2 and WSC+4
Hagun : 6*(TP + WS)
= 6*[ 5*(75 + fSTR)*pDIF1 + (75 + fSTR + WSC)*1.972*pDIF2 ]
= 30*(75 + fSTR)*pDIF1 + 6*(75 + fSTR + WSC)*1.972*pDIF2
= (2250 + 30fSTR)*pDIF1 + (450 + 6fSTR + 6WSC)*1.972*pDIF2
= (2250 + 30fSTR)*pDIF1 + (887.4 + 11.832fSTR + 11.832WSC)*pDIF2
Futsuno: 7*(TP + WS)
= 7*[ 4*(75 + fSTR+2)*pDIF1 + (75 + fSTR + WSC)*1.972*pDIF2 ]
= 28*(84 + fSTR+2)*pDIF1 + 7*(84 + fSTR+2 + WSC+4)*1.660*pDIF2
= (2352 + 28fSTR + 56)*pDIF1 + (588 + 7fSTR+14 + 7WSC+28)*1.660*pDIF2
= (2352 + 28fSTR + 56)*pDIF1 + (976.08 + 11.62fSTR+23.24 +
11.62WSC+46.48)*pDIF2
= (2352 + 28fSTR + 56)*pDIF1 + (1045.8 + 11.62fSTR + 11.62WSC)*pDIF2
TP : Hagun vs Futsuno Mitama
Hagun : (2250 + 30fSTR)*pDIF1
Futsuno : (2352 + 28fSTR + 56)*pDIF1
Futsuno win by : (158 + 2fSTR)*pDIF1
WS : Hagun vs Futsuno Mitama
Hagun : (887.4 + 11.832fSTR + 11.832WSC)*pDIF2
Futsuno : (1045.8 + 11.62fSTR + 11.62WSC)*pDIF2
158.4*pDIF2 vs 0.212(fSTR+WSC)*pDIF2
Assume total STR=160
fSTR=16 WSC=99
0.212(16+99) < 158.4
Futsuno win by 134.02*pDIF2