Okay, so I did some more precise testing on killer effects. I used Weapon Bash and Founder's Breastplate, which adds (killer rate/2)% to your damage.
First, to check whether Founder's Breastplate rounds or not:
Eschan Sorcerer, 3% killer rate from vorseals
Base: 303
Founder's Breastplate: 306 (+1%)
So, it looks like Founder's Breastplate rounds the damage down to the nearest percent.
Now for the meat of my tests. I did tests on both non-NMs and NMs. Arcana Killer II was the only base level killer effect active.
Sinister Seidel (non-NM)
Base: 364
Founder's: 382 (5% above base)
Circle 418 (15% above base)
Founder's/Circle: 468 (11.96% above Arcane Circle)
Circle (Enhanced): 425 (17% above base)
Founder's/Circle (Enhanced): 480 (12.94% above enhanced Arcane Circle)
Arcana Killer II on non-NMs is 10% killer rate.
14-15% rate for Arcane Circle
16-17% rate for enhanced Arcance Circle
Teekesselchen (NM)
364
374 (3% above base)
400 (10% above base)
428 (7.00% above Arcane Circle)
404 (11% above base)
436 (7.92% above enhanced Arcane Circle)
Arcana Killer II on NMs is 6%.
8-9% rate for Arcane Circle
10-11% rate for enhanced Arcane Circle
Due to the breastplate's flooring effect, I can't hammer down the circle effect further with these tests. Also, I don't have the numbers written down anymore, but they're much simpler tests due to not involving Arcane Circle, so I'll put this conclusion down too:
Killer effects from food are halved on NMs.
Tests to be done: check whether vorseal killer rates are nerfed on NMs (I've confirmed that they're not halved, but 3% > 2% is still possible with my numbers), and to use an odd value source of killer rates to nail down circle effects more exactly.