I also have just got the 1gb system. I have the exact same readings on the superhub and get very similar speeds. I have installed a cat7a cable what still shows the same speeds also if you use the superhub 4 as router you don't get your correct external ip address so it causes problems when setting external devices like cctv or nas
I, although currently at work am having similar problems and i suspect - lots of people will be - i am going to be looking into my own networking topology in the next few days
I have new cat-8 RJ45 (supposedly) cables ready to install to start trying to find the bottle neck
If your Superhub4 is in modem mode - i advise first checking that the cable between the Virgin router and your own router - is the highest spec you can get (cat 8 in my case) - then also check what wifi type your own router can send out (is it MI-MO), also check that the device your using is MI-MO too so that you can eliminate that problem...