Over the course of a 10 minute fight, if we run with the same assumption, then you get 8-10 Seigans.
10 Seigans
150 Kenki
2000 Potency
6 Shintens
150 Kenki
1800 Potency.
On average, approximately 2 extra auto attacks. If we assume that you can get at least 2 a minute, then we go with 20 Seigans and 12 Shintens as outlined above, or about 4 extra auto attacks over 10 minutes. If we assume 100% usage, then it's 8 extra auto attacks over 10 minutes.
If we value auto attacks at 2.5k on average, then over 10 minutes with 100% Seigan usage, you deal an extra 20,000 damage. This Samurai likely has a sustained DPS between 5500 and 5800 over 10 minutes. We will use the lower amount just to make this "look better" for Seigan.
5500 DPS over the course of 10 minutes is 3.3 million damage. 20,000 extra damage over the total of 3.3 million is .6% damage increase for 100% increased usage of Seigan. Someone actively aiming to use Seigan is more likely to hit around 50%, so .3% increase. Add in the potential for it to simply be missed via better priorities (not clipping GCDs, mid Iaijutsu, whatever) and it's more likely to be around a .15-.2% increase.
So effectively you're calling a 99.5% Samurai a baddy.
