http://forum.mikrotik.com/t/tcp-performance-over-mikrotik/56922/1
http://forum.mikrotik.com/t/nv2-link-with-a-very-different-speed-in-tcp-and-udp/78890/1
http://forum.mikrotik.com/t/wired-and-nv2-802-11n-multi-hop-throughput-issue/73484/1
http://forum.mikrotik.com/t/tcp-performance/61647/1
This problem is seen by a lot of people and nobody has a real solution until now. To reproduce it just put nv2 links in row and you will see that tcp speed decreases while udp speed stays up at the speed of the slowest link involved. This was not the case with nstreme (But with nstreme we’ve stability problems since 6.x releases).
I guess it has to do with how nv2 queues/aggregates packets. But this is just guesswork.
So while tuning nv2 for .ac please dont look only into a single link. Just put 3 links in a row (SXT AC are cheap you know 
)
and run a single tcp stream test end to end and see how much it differs from UDP speeds.