Thanks for taking the time to reply.
I've tried the sam knows on the SH4, after changing it back to router mode. It does show 1+ gig to the router - but from my 2.5gbe switch to my 2.5gbe pc it shows a variation of results (from 80mbps (!) to 600+) - I'm not sure I trust that bit of it if I'm honest. I've tried different cables, I've done internal network speed tests, all get the reported 2.5gb (minus overheads) - so I know the link between my switch and my pc / network is working.
I had an engineer come out and plug his analyser in, and it came back not at 1gig but well over 800mbps. He did say it was fairly new in our area and may take a while for the network to settle?
My only guess is that there's something funny going on between the coax and the LAN port internally of the SH4?
@Avalon thank you as well, hopefully the above helps.