The N can go up to 300 Mb/s but the maximum speed average is from 144 to 200 Mb/s.
Since your'e getting 270Mb/s, that's already the maximum speed,so no worries.
The reason why you only have 270Mb/s could be with the interferences. One way or another it won't stay at 300 Mb/s.
If you already have a dual band adapter, try using a dual band router.