Testing Acc/Eva/Def might be a bit hard with such a low %, but I'll try fighting cactuars for 1000 needles to see if we get any -DT.
Also, I got some killer affects to test on lizards, icecday, naked, all with Ferine Gausape +1:
w/ Panther Mask, Tatami Shield, Killer Shortbow, Beast Trousers +1
Stonega II 348 damage
w/ Panther Mask, Killer Shortbow, Beast Trousers +1
Stonega II 345 damage
w/ Panther Mask, Tatami Shield, Killer Shortbow
Stonega II 345 damage
w/ Tatami Shield, Killer Shortbow, Beast Trousers +1
Stonega II 341 damage
w/ Panther Mask, Tatami Shield, Beast Trousers +1
Stonega II 345 damage
w/ Tatami Shield, Beast Trousers +1
Stonega II 341 damage
w/ Beast Trousers +1
Stonega II 338 damage
w/ Killer Shortbow
Stonega II 341 damage
w/ Tatami Shield
Stonega II 341 damage
w/ Panther Mask
Stonega II 341 damage
With just Ferine Gausape +1
Stonega II 338 damage
Total nude
Stonega II 332 damage
Panther Mask's "Enhance Lizard Killer Effect", the rest are all "Enhance Killer Effect".
Beast Trousers +1 seem to give a bonus with certain pieces, not quite sure, but I repeated Beast Trousers +1 with Ferine Gausape +1 several times and got the same damage so I don't think its randomly activated.
So Killer Effect from equipment does increase damage, though Beast Trousers +1 still confuses me...
Edit#1: Added more tests.
Edit#2: Just tested in Desert against cactuars using a charmed pet for TP gain and lowered 1000 needle damage so I don't die.
w/ Ferine Gausape +1
1000 needles did 490 damage
w/o Ferine Gausape +1
1000 needles did 500 damage
490/500 = 2.00% -DT!
Edit#3: No merits. Tested intimidation rate. Vs. Mandragora in Saruta with Ferine Gausape +1.
Intimidation 4.23 %
253 intimidates out of 6239 hits.
Tested intimidation rate. Vs. Mandragora in Saruta naked.
Intimidation 4.06 %
222 intimidates out of 5688 hits.
Seems to be an increase in intimidation rate, hopefully this is a large enough sample. Fits with the (almost) 2.00% from the other bonuses.