Just an update on this. The Grandstream AP arrived, I ordered a power adapter, plugged it all in and immediately saw an increase of 200mbps up and down over the HG659b. We've seen as high as 675mbps down and 450 up over WiFi in the same room.
In the far reaches of our house, we now have more reliable coverage and I saw 250 down / 150 up when I tested it.
Really impressive results so thanks everyone for all of your help, especially michaelmurfy with the Grandstream recommend.