Did a few more tests without wind and no turbulences.
At first I thought they were inconclusive, because I got drops to 6000 with both settings and sometimes both would behave well (~4000-ish).
However during one test with a drop I noticed a slight stutter during the nose dive, which, I presume, was the result of either x-plane or an addon performing some loading action due to the view changing (my prime suspect is Skymaxx). Such a leap in the time continuum could easily throw off any PID controller.
So I disabled (almost) all plugins and relocated the testing area out to open see. So far I haven't seen any unruly drop in the vertical speed there. Haven't tried with turbulences yet and the number of test has been rather small - running out of fuel
However I currently think that drop is due to fluctuations in the physics, either in form of the frame rate (stutter or slow downs) or maybe wind speeds due to turbulences - not sure yet about the latter.
Edit: It seems turbulences (set to 1) can cause this as well - although it seemed to be a little less extreme (would barely touch 6000)
PS: Didn't know you can ditch planes in X-Plane - good thing the operations manual includes a ditching section. Drag seems a little low though.
PPS: Hope the life vests are included with IXEG's model.
PPPS: Send boats... or nudes... or both...