If you upgrade to 7.x (if possible at all) you definitely lose NV2.
I don't see any reason to upgrade, the couple of devices is a wireless bridge, no need for anything that has been "bettered" in v.7.x AFAIK, and the speed of the link is more or less the same, according to this report:
CubePro Perfomance
At 300 m distance you should get very decent speeds.