This is because you have not configured your router correctly. 802.11n ist able to transfer 150Mbps per stream. If you configured your wireless to allow operation in mixed mode, than there is only one stream for 802.11n left, thus if you want to get 300Mbps you have to set Wireless Network Mode to "N only" + "Channel width to 40Mhz"

In N only mode your g devices cannot connect. If you need mixed mode than the max transferrate will be 54Mbps for g and 150Mbps for n devices. Because some of the antennas will be used for g transfers and some for n, therefore you cannot serve multiple n streams which are needed for full 300Mbps.

What wireless card do you use?
You are using a wnr3500l router, right?

I have an Atheros AR9285 Wireless Network Adapter, and a WNR3500L with tomato on it! Tried channels 1, 6 and 11, it still only connects with 150Mbps. Was that really all you did to make it work Ali?

About the channels, is it a good idea to set it to a channel that other APs in the vicinity does not use, to avoid interference? Also, does 300Mbps require Ipv6 to be enabled?

I would really like to get this problem fixed to increase my wireless transfer speeds. Would really be nice when using files on my NAS.

Thank you Kong, that explains it. Too bad.