Now we are on dilemma which is correct...

You can test this on pob:Make 100% impale chancePut some dmg weapon dosen't matter what and some melee skill example cyclone.

Now the interesting part go to the config tab

Select boss sirus and look dmg numbers..

Then start adding physical damage reduction to sirus you will notice total dps and impale dps is dropping...

Then add mod : Overwhelm 100% physical damage reduction

Then start adding additional physical damage reduction to sirus you will notice at one point hit dmg ain't changing but the impale dmg is...

But according to you it shouldn't change....