This may be a bit off-topic, but I recently upgraded my Internet plan from 30 Mbps to a 150 Mbps plan, requiring a new cable modem. Before the upgrade I had been running the old cable modem in bridge-mode and using my Airport Extreme (802.11a/b/g/n) as the router.
When I received the new modem I tried to use the same configuration but could not achieve anywhere near the desired speeds the new plan offered. After a few support phone calls I eventually removed my Airport Extreme, had them unbridge the modem so I could use the built-in router of the new cable modem. I immediately achieved the expected 150 Mbps download speeds.
I would've liked to continue to try getting the Airport working, but since it involved phone support I couldn't troubleshoot the problem very much. As in your case, I know the Airport Extreme is capable of those speeds, yet something was holding it back.
I don't like using the routers in cable company supplied modems, but oh well.