Makes sense to block outdated clients.
As it goes I now have transfer speeds up above 100kb/s which is tolerable.
In case anyone else picks up this thread via a search I should list the steps I took, I suppose.
I've set port forwarding for TCP 13094, 6694, 2104, 2106, & UDP 7000-7200 and launched the updater with
"D:\Program Files\Steam\steamapps\common\city of heroes going rogue\cohupdater.eu.exe" -port 13094
I've also disabled (temporarily) stateful packet inspection at the firewall.
Might be worth adding, this is on UK Virgin Media Cable - ISP traffic shaping may be the root cause.