Indeed! Luckily, to date there is very little content available (none?) that requires ≥100Mbps of bandwidth to Vero. 
In this specific case, I am suspecting the culprit may be the powerline segment of my LAN. All other devices, and those happen to be upstream of the PLC adapters, achieve adequate throughput rates.
This is the setup, consisting of AVM Fritz!Box 6360 cable modem, D-Link DGS-105 switch and two devolo dLAN 1200+ / WiFi ac adapters with 1 resp. 2 GbE ports:
Router --- GbE Switch -+- Powerline Adapter --- Powerline Adapter -+- Vero
+- Ubuntu Server NAS +- RPi
+- ...
Either way, I am not actually expecting gigabit speeds over my power-line connection; Just some value beyond 100Mbps because the RPi achieved similar throughput rates. And we are trying to investigate the awful speeds occurring when configured for GbE. So if there is anything else I can provide that will help with this, please let me know. 