Your results are typical of most Wireless N scenarios. Please bear in mind, 300 is a “perfect world” “dreamland” number. This will never be achieved in the real world.
Try installing inSSIDer 2 on a laptop. It will show you exactly what modes your access point supports, given its current configuration, as well as frequencies and other neat stuff.
In mine scenario i get 65-144 mb trafic from RB2011UiAS-2HnD. Make sure that:
- You have selected “indoors” in advanced cinfiguration.
- You have enabled both channels.
- You are selection the most “free” channel
Also please note that, if router are declared as 300mb, you will probably see half of that, in my case 144 mb max i see. So if you see, 100+ mb connection via Wifi, then is OK.
300mb is teorically limit that can achieve via N protocol, but that is of course in area without other radios, inteferences etec.. In normal use, expect half of declared speeds. 
WMM is disabled in my configuration too. I dont know if enabling will increase performaces.