I have an Intel 7260ac PCI card that never seem to go all that fast under Linux.
The NetworkManager widget said it was on channel 149 and doing about 130Mb but in Windows it gets 650 to 780 Mb in ac mode.
Turns out the NM widget was lying it was on 2.4 (so not channel 149) and was capped about 130Mb. I tried forcing it to 5GHz but it just roamed back to 2.4.
Hmm.. found a simple solution block the MAC on the 2.4GHz radio. Now it connects at the same speed as Windows on 5GHz. Simple really and not an issue if you have separate SSIDs.