Running the instancing demo on an iPhone6+ I can get to 150k particles before dropping below 60fps. There's an obvious speed up about three seconds in where I guess the JITter decides to get serious.
So, even though there's no official support for asm.js on mobile safari. It still works surprisingly well!
I continue to be staggered at how well asm.js performs, and really glad Chrome has taken on optimising for it as well as Firefox.