This is the result from my SXT G-5HPacD link tests
I have reduce the tx power to 1
NV2 Ce
http://www.wirelessinfo.be/index.php/linktesten/sxtacpages/labtest-sxt-ac-nv2e
NV2 eCee
http://www.wirelessinfo.be/index.php/linktesten/sxtacpages/labtest-ecee-nv2e
Results not impressed. maybe too high signal?
Indeed I think so too.
I have taken down the radiated power to 1 (tx-power) this is verry weak
This determines also the bad performance
RouterOs 6.20 is not yet properly adjusted for ac configurations.
Soon put I link test of 500 meter online.
I wait behind good weather here in Belgium 
This is a 500 meter linktest with 802.11 protocol
http://www.wirelessinfo.be/index.php/linktesten/sxtacpages/802-500-eceee
And Nv2 protocol
http://www.wirelessinfo.be/index.php/linktesten/sxtacpages/nv2-500-eceee
Try with:
ht-rxchains=0,1 ht-txchains=0,1
Set to this way