While Intel keeps silently hiding in a bushes, it seems they have finally managed to fix slowdown problems with a driver version 16.8.0.6. But for some strange reason they do not share their latest drivers with anyone but Dell:
http://www.dell.com/support/drivers/us/en/19/DriverDetails?driverId=5VYHN&fileId=3334123740
Even if installer claims "driver is for Dell computers only" and would not install on my Asus ultrabook, it still unpacks AC7260 driver by default to C:\Dell\Drivers\5VYHN\Drivers\, from where it can be installed without hassles on any system equipped with AC7260.
I have not observed any problems so far with this version, it even fixed Bluetooth mouse disconnection problems (BT is integrated with WiFi in AC7260, it seems two technologies are tightly interrelated here). In particular I am no longer experiencing WiFi slowdowns when connected to 2.4GHz 802.11n AP in the heavily RF-crowded environment with 100+ active WiFi devices and ~40 different APs vith various SSID's within reach.
It is sad why the driver dated 05/12/2013 and released on 1/13/2014 (according to dell.com) has not yet found its official way to Intel's driver download page. Intel's fake claims on "we have solved them in driver 16.6" reminds infamous Pentium FDIV bug and Intel's long term denial on its existence. People and companies do mistakes, this is not avoidable. Daring to recognize them and standing behind them IS a dependable company attribute. But lies, hiding and denial is not. Shame on you, Intel.