FWIW I also noticed the same thing on R720 with newer ZD builds (forgot which 10.x version, but one of the 10.x's after 10.0 GA) and also vSZ -- vSZ is really easy to test. 3.6.2 is lightning fast, 5.1.x builds had the same characteristic downlink results where the speed start off good for a split second and then decay to 100mbit, but uploads were always in the 500mbit range.
In my experience this affects Wave2 APs the most (R720, R510 in my personal experience), in combination with any Apple or Broadcom based client. Seems like QCA clients and Intel Wifi clients are not as affected.
But yeah, I'd invite QA to try Unleashed 200.6 vs 200.7 against any recent 802.11ac capable iPad or iPhone (made within the last few years) with a static channel + 80MHz out of the box config. I've had this problem on 3 deployments.
With regards to hayk's regression testing, I agree too that 200.4 was the fastest Unleashed release with Wave2 APs. For me the drop between 200.5 and 200.6 was not very noticeable, but the drop in 200.7 was huge. I don't want to roll all the way back to 200.4 as Unleashed has improved in many many other ways in recent releases. It's just the downlink throughput has really suffered.
(P.S. I honestly think this is an upstream regression with your chipset vendor. I have some Meraki APs in my test lab and their Wave2 APs are exhibiting the same behavior with their firmware 26.x vs firmware 25.x, on 26.x speeds start 'good', decay hugely, then slowly inch back up. Based off public info in FCC diagrams, I see the R710/R720 and Meraki's Wave2 APs use the same underlying chipset....
P.P.S. 'good' for Meraki means 25% lower speeds than Ruckus unless standing right in front of their AP. Nobody comes close to Ruckus in antenna design!)