Ok, this seems to be a case of interference.

I have ruled out my 5.8GHz cordless phone system when not in use (unplugged all batteries and disconnected power from base stations).

I have also disabled my network filter while testing.

Interference robustness turned on can get my up to 4.7MB/s. This setting normally helps in noisy environments and says it can slow performance. In my case, since the interference is killing performance, this actually improves it.

I'm still 2MB+/s shy of where I'd like to be though.

Here's a strange one... Even if I connect only to the 5GHz radio, I am getting consistently better through-put if I keep the radio setting on the base station on AUTO which allows n connections at 2.4GHz.

How do I find out where this interference is coming from? And should I start to make plans to set up a faraday cage around my house?