There are a number of threads here on this topic and at least one on which numerous people have posted their results. Unfortunately, the search function on forums here is not all that good and it can be hard to find.
I have an RT-N16 running beta19 and I get about 7.5 MBps give or take 0.5 with reading a bit faster than writing. This is about the best I have heard of.
I have never seen a definitive answer as to why some people get much worse results. So this does not really help to improve yours - I would if I could - but tells you what is possible.