I was doing some research about the Killer+ effects now that we have significantly useful armor with the effect on it. I came across this post https://forum.square-enix.com/ffxi/threads/46182?p=573968&viewfull=1#post573968 .
It appears that Killer+ job trait only affects the probability for monsters to be intimidated, and nothing more. The jp wiki also seems to say the same thing.
Damage+/damage reduction- is mentioned on the bottom of the page, but says its only around 2% with a +10 bonus. That section references the Ferine Gausape +1 which says "Augments Killer Effects".
If anyone else has other testing about damage, please share. I don't know where the information on the BG page came from, so I added verification tags. It could have meant to read +5% damage max, and -5% damage taken max.