Apologies, it seems the super hub CAN handle N and G at the same time, so all N devices get full speed regardless of connected G devices.
Posts like this :
http://community.virginmedia.com/t5/...of/td-p/250089
And found a couple of others too.
And providing your on the same ring circuit yes you can use the homeplugs to re-locate. It's as if you were just plugging the router directly into the back of the superhub. But instead you are using an extension.
As mentioned before i would set the old router up as an access point then the homeplug cabling between it and the superhub will ensure anyone connecting to the old router gets DHCP allowing them to connect to the internet.