Thank-you for linking me to that thread bluepoint, but at this point (until I investigate more) I believe the 360 Slim is negotiating the wrong Link Rate and it has nothing to do with my router (details after the break).
My 2012 AirPort Express does almost exactly the same thing (see box below) and it has a completely different chipset
(Atheros vs Broadcom). The only difference is instead of connecting at 5.5Mbps it connects at 11Mbps.
So now there is more than one chipset having trouble. I really think this is a problem Microsoft has to fix.
And they won't unless enough people complain.
P.S. This is the second time I've had bad experiences with Marvell wireless chipsets. The first was the Marvell chip in the PS3, it refuses to work normally without CTS Protection on (even though every other device I have does).
360 downloading while connected to AirPort Express:What your looking at here with the AirPort Express is the link speed while connected to N is 11Mbps limiting downloads to about 800KB/s. Yet when forced to B/G the link speed is 54Mbps letting the download go much quicker (1.5MB/s).
In comparison the my RT-N66U link speed while connected to N only manages 5.5Mbps and about 400KB/s. With the RT-N66U set to G it manages a 54Mbps link and about 2MB/s downloads.