Am I going to get 300Mbps from my 802.11n AP?

The standard answer is "it depends"!

So how?
First, you need to configure your AP to work in 40MHz channel bandwidth.
Second, you have to make sure that there is no other non-802.11n APs surrounding your AP.
Third, your laptop should use 802.11n device.
Fourth, there is no other non-802.11n devices connected to your AP.
Fifth, all the 802.11n device connected to your AP should use 40MHz and not 20MHz channel bandwidth.

So in reality, there is almost impossible to get the full speed of 802.11n in 2.4GHz spectrum. It may be possible for 5GHz spectrum.