the N speed is indeed 270 and not 300
Try to put it on WEP encryption and see if there is any difference, some drivers of clients don''t respond well to wireless N and AES
How did you tested the troughput anyway?
small note:
mbits = megaBIT per second, if you devide it trough 8 you get megabyte/s
mb/s = megabyte per second
the speeds of networks are usually displayed in mbits
so the theoretical speeds of networks are:
100mbit: 12.5mb/s
270mbit: 33.75mb/s
1000mbit: 125mb/s
note that all these speeds are commonly faster than harddrives, most harddrives only can handle 10mb/s