Improve throughput with Intel 5100 in wireless N mode

When I test its throughput (from a wired computer) in wireless N-only mode with iperf in vista, the maximum throughput I was able to get is around 25Mbits/s although it shows 270-300Mbps data rate by windows. The router is a Airlink AR670W with stock firmware 1.01 and 40MHz channel width/Auto(DFS) channel. It's not faster than the throughput of Wireless G mode.

After I update the firmware of AR670W to DD-WRT build 13069, I found the signal quality of the Intel 5100 client is only around 23% at full Tx power (251mW) from the wireless status page. And it can only improves the throughput to ~35Mbits/s with iperf -w 64k -c intel-5100. Then I started to adjust the following things:

1. Orientation of the two antennas of AR670W: On one position I found the signal quality of Intel 5100 was improved to 34%. 
2. Manually set the channel number: After doing a site survey, I found the least crowded channel was not the one being picked up by DD-WRT. So I changed the channel number and the signal quality of Intel 5100 was improved to 40%
   
   Then I tested the throughput again, and no surprise it's now ~80Mbits/s. Even if I further increase the TCP windows size of iperf from 64k to 10M or reduce the Tx power to half (126mW), the throughput remains at ~80Mbits/s (wired to wireless). Later I had to enable the N-only wireless mode on a D-link DIR-655 router to maximize the throughput of another Intel 5100.