I have an Airport Extreme from Apple and when I do a file transfer from a hardwired windows desktop to the Surface I rarely get speeds above 5mb per second, its usually between 2-5mb per second like you. I switch between 2.4 and 5GHz and see little difference. Could it just be that the surface isn't capable of processing/writing/receiving files any faster than that?
On my old router I was getting 7-8 MBps on 2.4ghz Wireless N and thats with many wireless networks around me. I'm getting half that now on 5Ghz and I'm the only 5Ghz network in my area. I'm thinking it's this new router, I'll probably return it.