The powerline/homeplugs are your problem, especially if there is a lot of noise on your mains wiring. Under laboratory conditions the best one direction speed you could get is less than half the rated throughput. Now transfer that to real world conditions and you can cut that by 50% to 70% and, in very noisy conditions, even more. There are several other factors about power line adapters that have to be taken into consideration as well - efficiency and throughput is proportional to price is the main one, don't expect good speeds with cheap units. Another is the quality and topography of the mains wiring.
Having shot down your network may I suggest that you try and construct your network with cable and where that is impossible, good wireless APs.
A little ingenuity can usually hide the cables in most homes. There is usually room for several cables just under edges of fitted carpets and there are several trunking systems that do duty as skirting boards.
You can also do a simple test using cables from the DNS-320 to your computer either via the Super hub or your netgear router and see what speeds you get with that setup.