I have just signed up to the 200 VIVID Gamer broadband with the SH3. The service has been running for about 4 days, and I'm not seeing more than 60Mbps download at any time of the day. Upload is fine. I have tried restarting the SH3 etc...
I'm also seeing quite a lot of disconnections when playing Elder Scrolls Online (Xbox one). Definaterly not xbox live. All devices are connected directly to the SH3 via ethernet cables, no wireless.
There's a couple of things to do to ensure that the speeds your getting aren't being caused by anything else.
If you head over to here and set up a Broadband Quality Monitor that will help highlight some issues you could be facing, it's used a lot on these forums and can show instability in an easy to view graph.
Secondly, the speed tests. Are they the XB1 speed test results or from a PC? The XB1 speed testing isn't what I would rely on as there's not enough detail to go off.
If it's a new install, the power levels could be to blame but I'm not an engineer so that would be something a VM technical support representative would have to get involved with. Have you spoken to them on the phone yet? They can normally tell very quickly if the power levels are too high etc.
When you get disconnected, do you lose connection completely, like the SH3 has lost the connection as well or is it just to the game servers?