Maybe I am wrong but isn't the intel x540-t2 only capable of 10 Gbe - max speed of 1-1 1.2 GBps and this under specific circumstances.
As far as can understand - although the switch is usung port trunk and showcases 80 Gbps the speed to the Qnap is limitted from the intel nic, which supports only 10Gb, maybe 20 if both ports are used with Link aggregation, which is a story of its own.
Another colleague, living in different country had to switch from RJ46 copper cable to SFP. Regardless of patch cables he tried (CAT6a, CAT7, even CAT8), he never managedto get sustained speed with his small 10Gb network setup. He eventually swapt the copper NICs to SFP ones and shares different experience.
Since You mentioned that rendering in DR does not actually engage the network in full capacity, think the issue is within the file formats which are being transferred over the network and the software environment. Several hundred jpeg files with file size of up to 10 Mb from random locations is overkill regardless of system. Have you tested the setup and speed if you copy single 1-5 GB file (psd, exr) from the Qnap to all computers simultaneously - do you get the very same network speed?