This feature will work just fine with netmap if done correctly. It’s documented here already: http://forum.mikrotik.com/t/how-to-edge-router-and-bng-optimization-for-isps/150007/1
Plus with the above method, you can also ensure layer 4 protocols such as SCTP, DCCP, UDP-Lite will work along with P2P networking such as BitTorrent seeding etc.