not sure what you are trying to say, but I have a single band Wireless N adapter on my laptop and desktop computer and I get 300 Mbps quite often on both of those machines.
whats the bandwidth show within 10 feet of the router using 5GHz N? 150Mbps = 20MHz / 300Mbps = 40MHz but only if your wireless client is a 2Tx/2Rx(dual stream), if not best you will see at 40MHz is 150Mbps(single stream client). If it is truly an dual stream client and highest bandwidth is 150Mbps then it is only operating at 20MHz.
lol single stream...theres a difference, half the bandwidth difference.
your confusion with dual/single band and dual/single stream: dual band clients(2.4 and 5GHz)
cannot use both bands simultaneously, some routers support simultaneous use of both bands, but not clients. dual stream is the use of two or more radio antennas called MIMO to increase performance usually by double, a single stream is like the old original wireless A,B,G with one antenna (54,11,54Mbps) speeds and now wireless N (75Mbps), adding in the second antenna using MIMO can double A,B,G, and N(2.4 or 5GHz) speeds (108,22,108,150Mbps). MIMO was around before wireless N if you didn't already know. both router and client must use MIMO for it to function.
with wireless N we also get channel bonding, all wireless A,B,G uses is 20MHz, wireless N uses 20MHz and can also bond another 20MHz together to make 40MHz, using that 40MHz wide channel we can double the performance once more above MIMO or single stream. dual stream MIMO 40MHz gives us 150 + 150Mbps for each 20MHz channel equaling 300Mbps. single stream 40MHz gives us 75 + 75 for each 20MHz equaling 150Mbps.