So the point of this post is to just document some findings about Killer Effects (KE), Killer Instinct (KI), and other Circles. Nothing ground breaking, but clears up some stuff that isn't documented.
Summary:
Sprits Within is great for testing this stuff.
BSTs benefit from each circle's +15 KE contributed to Nukumi Gausape +1 damage and a 1.15% damage multiplier.
Killer Effect merits only seem to affect native BST killer effects (Aquan, Beast, Vermin, Amorph, etc...) They don't help out with other circles.
At least Relic +3 hat works with all circles, I haven't checked other items, future todo.
Undead (Used Malevolence for +10 undead killer), Circle from PLD:
HP SW DMG Ratio WSD Nukumi Gausape +1 Holy Circle Expected Ratio Notes 2078 2285 1.10 1.10 1.00 1.00 1.10 +10 wsd 2076 2389 1.15 1.00 1.00 1.15 1.15 circle 2076 2554 1.23 1.00 1.07 1.15 1.23 circle + Nukumi 2087 2340 1.12 1.10 1.02 1.00 1.12 nukumi +5 killer +10 wsd 2087 3034 1.45 1.10 1.15 1.15 1.45 nukumi circle +15 killer +10 wsd 2087 2455 1.18 1.10 1.07 1.00 1.18 nukumi +15 killer +10 wsd 2067 2170 1.05 1.00 1.05 1.00 1.05 nukumi +10 killer 2007 2007 1.00 1.00 1.02 1.00 1.02 nukumi merits (looks like merits don't work) 2007 2107 1.05 1.00 1.05 1.00 1.05 nukumi merits + 10 killer 2027 2385 1.18 1.10 1.07 1.00 1.18 nukumi merits +15 killer +10 wsd 2027 2947 1.45 1.10 1.15 1.15 1.45 nukumi merits +15 killer + 10wsd
Crab: (Minor Correction)
HP SW DMG Ratio WSD Nukumi Gausape +1 KI Expected Ratio Notes 1854 1983 1.07 1.00 1.07 1.00 1.07 merits 1845 2439 1.32 1.00 1.15 1.15 1.32 merits KI 1989 2088 1.05 1.00 1.05 1.00 1.05 (erased merits lol)