Performance: I'm not convinced it makes sense to do this because the game might not reach 60 FPS. Modern mobile devices are about as powerful as laptops. Having one master switch to drop to 30 FPS on all devices is overkill IMO. I stand by this aspect of my previous post: if you are doing this to improve the user experience, then dropping a device that could easily reach 60 FPS to 30 FPS actually degrades the experience.
Here's the thing, let us be the judge, if the tool can do it, let us decide what's better for our games. It's an entirely different matter if the engine cannot do it, in this case, it looks like a browser conflict is preventing you from getting it done, as I said, fair enough.
Battery life: I think the argument here is better, although I would be interested to see actual data on the battery life savings. As far as I'm aware the screen is one of the biggest battery drain on mobile devices, so if the screen is on for the same amount of time at the same brightness, that part of the battery drain is identical regardless of how intensively the game is running.
Mobile SOCs loaded will burn through the battery much faster than the screen display. Example that really highlights this: play a video, an iPhone 6 Plus lasts for ~8 hours or more. The screen is on, but not much of the CPU/GPU is being used (just a dedicated decoder part of the SOC).
Now, load up an intensive game that loads the CPU/GPU, something like XCOM, watch that iPhone 6 Plus dead in battery in ~2 hours, while running very hot.
For the record, my game (link in signature), stripped of many effects, lowered on particles, removed shaders, optimized AI, on the iPhone 6 Plus Safari drains the battery in ~2 hours also. The PC version runs 60 fps on old Intel Graphics laptops from 2007. According to cpuutilisation, on my PC it loads ~20-35% of the single-thread. On iPhone 6 Plus Safari, it's ~60-90%, so it's hitting peak CPU usage during big battles.
Mobiles have incredible specs, on paper. But as soon as you try to extract peak performance out of them, their slim designs mean they get very hot and power usage increases. This is true for all mobile games, where developers never aim to fully load the SOC due to this reason.
This is a major concern for mobile gamers, if it's possible to lower CPU/GPU load % to achieve a steady state fluid experience, then it's something beneficial.
Would I trade 60 fps and get a steady 30 fps if it means my mobile device is now just warm, not hot, and battery life is near doubled? Yes I would. But give me that choice if possible.
Again, thanks for trying and looking into it. Sadly it's inherently linked to browsers and Chromium is the issue.
How does Safari treat vsync or half v-sync?