Thank you for being still with me on this.

I've downgraded to stock packages provided by Arch stable repository, which=
 is:
xorg-server 1.20.5
xf86-video-amdgpu 19.0.1
mesa  19.1.4
stock (read: no) xorg config
no custom kernel parameters (except of of disabling intel_pstate, see new
dmesg.log attached)

And I've also installed amd-staging-drm-next (6c7a8d5c0772) just like you.

But: The issue is unchanged. To further illustrate the issue, I've recorded=
 a
capture of it in the game TESV: Oblivion in D9VK (no difference to WineD3D =
or
Gallium Nine regarding this issue).

The capturing process in OBS Studio via Xcomposite breaks pageflipping, but=
 I
can turn it on again via TearFree which I enable via hotkey on the fly. The
result 100% matches running the game with modesetting DDX or amdgpu.dc=3D0 =
(no
spikes) vs. xf86-video-amdgpu + amdgpu.dc=3D1 (nasty spikes).

Hitman 2 is a bit different, as it doesn't show render spikes for me either=
 (I
think I was first mistaken regarding that difference, sorry for the confusi=
on),
but the mouse input just blocks/skips heavily and is even more unplayable t=
han
Oblivion/Skyrim etc.

I was just writing this while I read your new reply. I'll gladly try what y=
ou
have suggested.