There are a number of limitations that stop extra fast speeds on 802.11n.
1 antenna = max 150 Mbps
If you are in a busy area (like suburbia, 2.4 Ghz is used by more than just WiFi too) you will not get the 40 Mhz bandwidth for 150 Mbps, it will drop back to 20 Mhz bandwidth which means 72.2 Mbps maximum.
3) Guard Interval
If the distinct transmissions between between both units are not making it through, the Guard Interval will be doubled from 400 nanoseconds to 800 nanoseconds. Maximum now possible is 65 Mbps.
Most people will never see more than 72 Mbps on one antenna.
More: Wikipedia: 802.11n, "Data rates"