I've searched this forum but didn't find any threads, there has been a huge discussion over at forums DOT smallnetbuilder DOT com/showthread.php?t=7774 about the issue.
Basically when the xbox 360 is connected to the RT-N66U on shibby's build it's throughput is extremely slow which creates issues when streaming media. The issue has been fixed in the official Asus build (not sure which one it was fixed in) and works on the latest version 220.127.116.11.374.257. It has a special button in the wireless settings that is a tick box for "optimise for xbox" - I'm not sure what this actually does although i thought it set the 2.4ghz to N mode only (i was wrong).
The problem was identified as a problem with the broadcom driver. I was wondering if the issue is known by the tomato community and wether a fix is possible by commands/settings etc? or if in future versions it will be fixed?
I would also like to give a huge thanks to polarcloud, victek, toastman, shibby and everyone else who has contributed to what is by far the best router firmware to date.