Heya Fraser,
Appreciate the response. That's fair. Although I can only speak to my experience & I have noted tangible differences in the firmwares. I've had a mission where I suspected it was ISP though the engineer that came to my home did the line test through the XR1000 & from their Virgin Media Hub 5. Full throughput on the router & as I mentioned half that rate from XR1000 lan port (when I opened the NAT type things did improve but it's depressed in comparison to prior firmware with secured NAT. There isn't a lot of flexibility the ISP's router fimware.
I'll apologise I'm not to familar with DMZ settings or where I might get relevant addresses and figures for them. However, ISP router is in modem mode. Previously I did have my Nvidia Shield on the ISPs router & since reconnected it to the XR when I noticed it. It hadn't changed anything that I've noticed
Also I did have QoS set to never & any Congestion Control distributions reset for this troubleshooting.