I have the same problems. My Win8.1 system, updated the driver, and the result was super poor performance. Now, sitting 4 meter from my router, I barely hang on, and have max 15 Mbps... and 3 bars...
I tried to install the newest drivers, I tried to roll back drivers, but nothing helps. So I suspect, that when the problem happened, the driver updated a file, that is not erased when rolling back, or installing new drivers...
It has been driving me insane, especially, after reading this forum, I understand, it is an old problem, Intel has not solved.
However, I fixed it myself today.
My previous laptop was damaged, which is the reason I bought a new. But, the old still had the old wifi card, so I mounted this in my new Lenovo, and with a Qualcomm Atheros AR5BWB222 I now have 5 bars all over my house and 300 Mbps !!!! And that is even with Microsofts standard driver
So instead of being frustrated, may I suggest you just all change make of the WiFi Card ? life is too short to waste on a WiFi card that cost 40 dollars