I think that's isolated in Linux drives since I get zero latency and constant 866.7 Mbps in my Windows 7 laptop.
Could be, but I spent some time going through the source code for the Linux iwlwifi driver and didn't see any reference
to tx-rate stepping. My feeling is that it is managed in the iwlwifi-7260-9.ucode firmware, and that comes from Intel.
Well, it turned out to be that Intel firmware! A bit of searching showed the latest version to be iwlwifi-7260-12.ucode, which requires a recent Linux kernel. So, I installed a 3.19 kernel (from Debian experimental) and added that version 12 firmware from kernel git sources to /lib/firmware, rebooted and things were fine.
What specifically has changed? While the 7260 AC adapter still reverts back to its idling speed of 6Mbps fairly quickly (presumably to save battery power on laptops), it now also reacts more quickly to activity. The response is positively snappy when power-save is switched off, just doing some typing in a remote admin session is sufficient to achieve the maximum speed of 866.67Mbps, so no more irritating lag! That's called progress.