So I've had this issue since the October 2012 dashboard update I was beta testing for and it seems that in games like gears of war, shoot many robots, etc. (games that seem to have delayed action to when the shot actually "hits" your opponent) are laggier/higher in latency than connecting my same console by my home network's Wireless N.
There's no reason for wireless to be "better" on paper than wired, but one way I know for sure that the wireless in the console is not corrupted like the Ethernet adapter, is when you test your xbox live connection on the dashboard, Every so often, i get a NAT error until i retest and retest and it goes away. Using wireless, I've done the test 10 times in a row and not once do i get the NAT error unlike every 2nd or 3rd time with the wired connection.
My router isn't the culprit as my PC is hardwired which I have acceptable latency on the games I play. But on my Xbox 360, the connection types are night and day.
My 360 is not under warranty anymore so I'm not finding it worth the limited funds I have to send it in for this small problem, but if anyone else feels like they have this issue, please share. I believe there is a problem with the ethernet adapter's driver on the console that isn't fixed by system update or system rollback. So unless it was fixed by the factory, there's obviously no way to fix it from home.