Let's take a look on Navrey, a keyless peerless with 160 barding difficulty:
Normal:
View attachment 42406
Discorded:
View attachment 42407
She lost a flat 14% in each resist (each resist had 14 points subtracted from it), and her skills, sans poisoning were reduced by 14% (initial skill was multiplied by 0.86).
But she doesn't display the "shrugged off" message. So, for peerless bosses like Dreadhorn, Mel, Chief Paroxysmus, etc, a flat reduction of 7% in each resist and a loss of 7% on each skill (initial skill gets multiplied by 0.93) would be pretty acceptable.
So the discorded Dreadhorn would look like this:
Resists 39/54/56/63/67
Skills: 120.9/83.7/102.3/120 (posioning isn't affected by disco)/102.3/102.3/102.3
Much less than what Navrey suffered, but still worth the trouble of discording him.
As it is now, Dread, Mel, Chief Paroxysmus and so on are suffering a 1% flat reduction in resists and the initial skill is being reduced by 1% (skill multiplied by 0.99). Not a very return for your investment in bardic skills.