The speedtest client can only see the user data, not the encapsulation overhead.

An ethernet frame carries 1500 bytes payload, and is about 1536 bytes total by the time you add ethernet headers and framing.

If you are using PPPoE (without baby jumbos) then that's 8 bytes taken, leaving a 1492 byte MTU.

A 1492 byte IP datagram used for TCP carries 1444 bytes of user data (20 bytes IP header + 28 bytes TCP header, including TCP timestamp option)

So of the raw 330Mbps stream, the rate of user data carried is 330*(1444/1536) =~ 310Mbps

..... and what was the download speed with the Linksys ?

Thank you very much, that definitely answers my question

I don't recall it being any different, so it went straight back to Amazon, sorry I can't answer the question in a quantifiable way.