Question for the Samurai Vets

[Hezhi]

Do you mean as opener? When you open on a target, you want to be using Meikyo Shisui, into Kasha/Gekko, it will grant you your buffs instantly.

After the opener, on single target, 8GCD is better. To give you an idea, Hakaze is already worth almost 2 AOE combos in potency, and the 3rd hit of your single target combo deals almost twice as much damage as a 2hit AOE combo in one GCD. Shoha potency isn't high enough to compensate for that. So single target abilities are much superior for single target.

If you want to go more in depth:
-If there are only 2 enemies, 6 GCD is slightly better, but you should still be using your single target spenders (Midare Setsugekka, Hissatsu: Senei, Hissatsu: Guren). Also, if there are only 2 enemies, Meikyo Shisui charges are better spent on Gekko and Kasha rather than the AOE abilities. And consider applying your Higanbana DoT on both targets if you know they'll stay alive for the duration of the debuff!
-If there are 3 enemies or more, 4 GCD is better (omit the Yukikaze + Hakaze), and everything is better spent on AOE abilities.