It is a cheap adapter I purchased from dx.com.
I am on Vodafone VDSL
If I run speedtest.net I get approximately 8.5 down and up.
If I put a Windows 8 Laptop (with its own RJ45 connector) directly into the powerline box I get about 35 down and say just over 9 up. This is what I get when I have the Windows 8 laptop in another location, but connected to an access point, which in turn connects to a powerline box.
If I purchased (say from a NZ Retailer) a USB 3.0 RJ45 Ethernet Adapter, would you expect me to get much a much better download speed?