Samurai - Kaiten Issue

[Quor]

But isn't it better to spend the Kenki on Shinten instead of a combo finisher?

Shinten is 320 potency for 25 Kenki, which when divided out gives us 12.8 potency per Kenki, or 64 per 5. This means Kaiten has to give a minimum of 256 (64x4) potency in order to equal Shinten. At 480 potency on Gekko/Kasha Kaiten grants 240 additional potency, so you're still better off saving that Kenki for Shinten instead of using it on a Kaiten there.

In other words, you're correct.