I have a wierd anomaly and I'd like to know if there's a hardware limitation in the Intel adapter that's causing it.
In short, my 5100 can have RX (receive) rates of only up to 150 Mbps, while the TX rates can reach up to 300 Mbps! That's quite funny considering that usually it's the other way around... I check these rates inside the Tomato frimware on the router to make sure these are the actual rates that I'm using.
So it is clear that there's no problem with using 40 MHz wide channels, because I do use them. Plus, in another laptop with a 4965, the connection is at 300 in both directions.
It is (probably) not a driver issue either: I've updated to the newest drivers on my Win7 x64, and I'm pretty sure that my Debian Linux has the newest stable firmware & driver package. Under both OSes I encounter this issue.
Another thing that you might like to know is that the network is using 5Ghz frequency (channel 120 with upper control sideband).