OK, I thought I would put together another one of my nerdy tests to see which was best - Box 2D Web or asm.js physics.
Here's the test: [attachment=2:35dl1rrj][/attachment:35dl1rrj]
And here are screen shots of the results, run on Chrome stable Version 39.0.2171.95 m, W8.1 x64.
[attachment=1:35dl1rrj][/attachment:35dl1rrj]
[attachment=0:35dl1rrj][/attachment:35dl1rrj]
On Firefox, both asm.js and Box 2D Web topped out at 701 objects / 52 FPS.
Is it just me, or does asm.js seem to offer no discernible improvement over Box 2D Web (at least for this bouncing ball test)? Subjectively, there was no difference between how each test appeared to run on my laptop. There might be situations where asm.js offers better performance and I would be interested to learn what these might be, but I don't see any evidence of that in practice.
In light of these test results and the apparent stagnation in asm.js physics development, is it worth dropping asm.js and then implementing full Box 2D physics?
Edit to tag Ashley and to say +1 to all of the suggestions above.