folks, you need to understand how 802.11n works. the ipad has a low power 802.11n adapter. it can only use up to mcs rate 7 instead of 15 like your laptops/desktops. this means you will never see more than 150Mbps connection and that would only be if connected to the 5Ghz radio of your access point.
if you see a max of 270 on any device it sounds like your access point doesn't support short guard interval. your ipod connects at 54Mbps because it's only a b/g device. and if your ipad connects at 39Mbps it appears to be using mcs rate 4 and a 20Mhz wide channel. so it sounds like it's connecting to 2.4Ghz radio.
dual band radios depend on the software driver to determine how they will connect to an access point and what band they will prefer if both 2.4 and 5Ghz is available. perhaps the ipad's driver chooses 2.4 first and then 5Ghz only if 2.4 isn't available? if you can disable the 2.4 radio altogether and test the ipad. problem will then be that your ipod won't connect. this is where the newer access points can help if they can run both radios and use separate ssid's for each. that is a way to force your clients to use one radio versus the other.
http://www.ciscostadium.org/en/US/solutions/collateral/ns340/ns394/ns348/ns767/i mages/whitepaper_80211n_design_and_deploymentguidelines-1.jpg