I have a Dell Latitude E6510 with the Intel Centrino Ultimate-N 6300 and a Netgear WNDR3700 acting as an access point. I was using a clean install of Windows 7 64-bit with the drivers off the Dell Support site and was able to connect on the 2.4 Ghz and 5 Ghz network at 300 Mbps. The problem was maintaining the 300 Mbps on the 5 Ghz connection, the link speed would jump around almost randomly regardless if the maching was idle or copying large files over the network.
I installed the PROSet v126.96.36.199 package and now when I connect I see a notification that indicates it connects at 300 Mbps but it immediately drops down to 150 Mbps on 2.4 Ghz and 90 or 120 Mbps on 5 Ghz. It seems to be 'disabling' an antenna regardless of how I am using the network.
Firmware on the WNDR3700 is V188.8.131.52NA
In my own troubleshooting I think I've fixed this. In the PROSet WiFi Connection Utility Details window it now says "Number of Antennas in Use 3" as it should
I used 'Reset PROSet' from the tray icon and now its working as it was before: 300 Mbps on 2.4 Ghz and usually less when connected on 5 Ghz. .