I read some articles that stated the new 3x3 AEBS and TC were only capable of 300 Mbps, those reporta are wrong, I bought a new AEBS and have that setup as my new router and I have a new TC setup to extend my Wireless N network they are showing in the Airport Utility speeds between 400 and 450 Mbps look at the first device in the list.
http://att.macrumors.com/attachment.php?attachmentid=200211&stc=1&d=1256419609
http://att.macrumors.com/attachment.php?attachmentid=200211&stc=1&d=1256419609