wnr3500L v2 + shibby rt-n 116, 300 megabit same as 150

Hi everyone. I noticed that I get the exact same performance with a 150 megabit wireless adapter as with a 300 megabit one (10-12 MB/s). I was wondering why that was. I'm running N only, wpa2 personal aes encryption, 40 mhz on 2.4ghz ( I know that's not ideal), manual wlan interference mitigation, with frame burst enabled, apsd disabled. Does the firmware or any of these features have a 100 megabit wireless bottleneck? Or is 300 megabit not fully supported for this device? The device list...

wnr3500L v2 + shibby rt-n 116, 300 megabit same as 150