Very interesting - I think you are correct !
Not only the built-in wifi is unable to switch to 802.11n - it also disconnects pretty often.
Quite infuriating when a lot of your time is spend on SSH console 
(I do use screen/tmux/mosh, but it’s still pretty annoying)
So yeah, I think the wifi module in my laptop is defective.
I think I’ll just use the USB wifi dongle, and use the mPCI port for something else…probable eGPU #yum