Reg, cable test result is not quite accurate and it says it is just for your information in the user guide.

Try to update driver for your ralink dongle. As I know, TP-LINK also have some adapters using ralink chip. You may ask them to test. http://ticket.tp-link.com/index.php?/Tickets/Submit

Your laptop and samsung f6400 cannot detect 5GHz Wi-Fi, which may be limited by themselves. If they don't support dual band but 2.4GHz only, they will not be able to detect 5GHz Wi-Fi. Although they...

C5, C7 V2 and C8 etc. improved the compatibility with DLNA devices and enhanced the overall performance. That's all I know.

Follow the following guide to check your settings. http://www.tp-link.com/en/article/?faqid=771

With the first W8960N, do you have the same problem? You'd better test WLAN to WLAN speed using some other wireless clients to make a comparison.