Hello again,
There are only 3 20Mhz channels that don't overlap (each of the 11 channels are only 5Mhz each). 1, 6 and 11. Each STANDARD 150Mbps channel is 20Mhz wide. So, knowing this, Channel 1 occupies channels 2 and 3 and also spectrum below channel 1(it is legal), channel 6 occupies channels 4-8 and channel 11 occupies 9, 10 and spectrum above channel 11, once again, this is 20Mhz channels.
Now consider a 40Mhz channel (2 bonded 20Mhz channels). If you choose channel 6 and a 40Mhz channel, you will either be wiping out channels 1-8 or channels or 4-11.
It is confusing for most people to understand when they choose channel 1, they also choose channels 2 and 3.
Now the good neighbor policy implemented in 802.11N in the USA says if there are people on other channels that you would interfere with by selecting a 40Mhz Channel, it won't let you select a 40Mhz channel, by NOT letting you select a 40Mhz channel, it is limiting your speed to 150Mbps or below(depending on many things). The hardware is working as intended by law. Being most people don't use 5Ghz(yet), you should have all the bandwidth you need up there.
The overall problem, when 802.11 was allocated spectrum many years ago, the speeds we use today were not considered, so they did not give us enough spectrum, thus today our channels overlap.
The Dir-825 will indeed do 300Mbps, my new home only has 1 WiFi user in the neighborhood, so I can select a 40Mhz channel and the router will give it to me. I also have another router that will allow me to force a 40Mhz channel and squish anything under it (it also goes to channel 14 instead of 11), but then again I also have an FCC license that is valid in the 2.4Ghz ISM band. But with that license comes the knowledge that by forcing a 40Mhz channel, it is BAD neighbor policy and I could cause interference to other people.
So, quite simply, you have too much interference from local users to do what you want, which is 300Mbps on the 2.4Ghz band.
The reason I advise people to use channel 1 or 11 is, most routers come default on 6, when you use 1 or 11, part of your RF use is outside the band, less interference.
Here is a Wiki article, sorry if i was too technical, Radio engineer for almost 30 years.
http://en.wikipedia.org/wiki/IEEE_802.11