The test I would try is an iperf between just the powerlines with and without the virgin equipment plugged in. If you can get your old speeds without it plugged in and it only drops after the virgin equipment is plugged in, the newer speeds may require more ampage and power draw changing the scenario on the power circuit which the powerlines use.
Finally got round to doing it; testing from one Mac to the other using iperf3 just testing network speeds across the wired powerline.
server set using iperf3 -s
client set using iperf3 -c 192.168.0.60 -P4 -t 30
First result with VIrgin Hub 3 plugged in switched on and connected to network as usual:
[SUM] 0.00-30.00 sec 1.06 GBytes 305 Mbits/sec sender
[SUM] 0.00-30.00 sec 1.06 GBytes 304 Mbits/sec receiver
Then tested with the Virgin Hub switched off and unplugged from power and disconnected from powerline or switch by pulling out the ethernet cables
[SUM] 0.00-30.00 sec 1.10 GBytes 314 Mbits/sec sender
[SUM] 0.00-30.00 sec 1.09 GBytes 313 Mbits/sec receiver
Then finally just for good measure I pulled out the coaxial cable from the back of the hub too
[SUM] 0.00-30.00 sec 1.11 GBytes 317 Mbits/sec sender
[SUM] 0.00-30.00 sec 1.10 GBytes 316 Mbits/sec receiver
Looks to me like has very little impact on the issue.
Unless TP Link aren't really talking about interference leaking from the hub or coaxial lines as such but that there is an actual issue with the way the data transmits at higher speeds from coaxial/hub into the powerline, as that always halves the speed to 160-170Mbps now with the 350 service, whereas used to be 200+ with the 200 service.
Any thoughts?