I am pretty sure that the fault will be found to be with the adapter itself, and probably not solvable by a driver update. I have been having the same issues as many others. I have a Toshiba p55w and the WiFi works great in the same room as the router. I do have an Lynksys e4200, and it is not AC compatible. The router cost me a lot of money two years ago.
What I have noticed is that the Wifi works great in the same room, but put a wall in the way and performance drops rapidly. On the 5ghz band, I can put a wall in between me and the router and maintain performance, but the 5ghz signal doesn't travel far. For me it is not dependent on signal strength, as you can see in the posted pics.
Picture one is same room. Picture two is bedroom (which is really close to the router, but has a wall in the way). Ping times are great on each test. Signal strength is maximum on each test. The laptop is connected to the 2.4ghz band. I have tried all the workarounds, settings adjustments, and have tried the Microsoft and Intel drivers (all versions). And notice how only download speeds seem affected?
This just sucks. Sure I could fix the problem using only my 5ghz band at home, but I cannot use it at work (older router, access point down the hall) and hotels are a miserable experience. I do have a USB WiFi adapter that makes the laptop usable, but I shouldn't have to have a two inch stick sticking out the side of my 1000 dollar laptop.
