Hi, I also have T8E with win 10. Did not have chance to try Win8 with 5Ghz, but after updating to Win10, the card has problems all the time.
2.4Ghz signal is good but 5Ghz is very poor, 1-2 out of 5. Router is only 5-8m away, other devices all have 5Ghz signal to the max, even next to the T8E.

Try this list for windows 10 wifi adapter. May be useful to you. Link: http://www.bestoninternet.com/compute/computers-accessories/usb-wifi-adapter-for-windows-10-reviews/

I have the same issue with Windows 10 pro and the Archer T8E. With the Windows 10 drivers, the 5GHz AC signal is either a 1 or 0. The 2.4ghz signal is good. This seems really strange since my cheap AC wifi usb and AC intel wifi adapter gets a much better 5ghz signal. I am wondering why I paid more money for an inferior product.

“Windows 10 Cortana is responding quite slowly. Whenever I say ‘Hey Cortana’, it takes 10 seconds to pop up. When I ask a question it either says try again later or just looks it up on Bing. Sometimes it even takes 5 minutes to do a web search. Help!”
---A user from Microsoft Community
Operating Windows 10 system but sometimes you may come across the circumstance like above— Windows 10 Cortana respond slow. It’s really annoying. But take it easy, because in this post I’ll share a good way for you to fix the slow Cortana response.
Perform the following Windows 10 command to fix Cortana slow.