This is only a dual band card, and depending on many different things (frequency, router setup, bandwidth, etc....) it is very possible to not hit 300 Mbps.
http://en.wikipedia.org/wiki/IEEE_802.11n-2009#Data_rates
Have a look through there to see what...
http://en.wikipedia.org/wiki/IEEE_802.11n-2009#Data_rates
Have a look through there to see what...