Toyota did a little marketing trick where the same drivetrain in a Highlander had slightly lower torque and HP published, than the RX-350, for 87 and 91 octane recommendations, respectively.
Toyota tricks date back at least to 2004 Highlander, RX330 and Sienna which I recall from having checked these vehicles out at the Chicago auto show. Those vehicles shared the same 3.3L/5-speed drivetrain. Highlander and Sienna were regular fuel vehicles; the RX was premium "recommended." These vehicles had the same HP and torque specs except, oddly, the Highlander hit the 242 ft-lbs at 800 higher RPM as I compare them now.
Sienna was V6 only but with no AWD option at the time. The Highlander still had a 4 cyl. option. The RX was V6 only with the AWD option, inherently less efficient across trims sold compared to the others. Recommending premium on the RX was an evident attempt to get that model's mpgs across trims up a scootch, maybe 1+ mpg while RX buyers would be more inclined to pay up for fuel. I bought the Sienna.
On the subject of altitude driving, I was running 89 octane/10% ethanol in the 2004 Sienna because it happened to be cheaper than 87 octane ethanol free where I lived at the time. On a road trip while climbing from the flatlands toward Yellowstone it stated pinging under load, accelerating uphill. I switched to premium and the ping went away within a tank. The vehicle went another 125,000 miles of trouble free driving before trading it in.
Now, with my 2014 Sienna and the next gen drive train (3.5L/6-speed), I run 87 octane/10% ethanol and when climbing to 5500 ft. crossing the Appalachians twice per year I've never encountered a ping. Since 2014, Toyota had added direct injection, increased HP and had gone to an 8-speed trans until they went hybrid only in the Sienna.
The point of the story is that over 10 years and a next gen drive train, the Sienna, et. al. engine and software underwent further refinements to evidently include anti-ping controls at altitude. Despite hiccups with one maker's model or another, internal combustion technology across the industry has advanced since that pinging Sienna from 17 years ago.
If I were the OP driving to 6,500 ft., and I was accustomed to happy driving on regular fuel for economy's sake, I'd keep using regular fuel unless there is some specific evidence to the contrary in the Mazda turbo. If it does happen to ping or show an unsatisfying power loss, then I'd make the switch.