Can't reproduce with the latest Chrome. I'm not really interested in testing old node-webkit versions, since Chrome updates usually improve performance and has focused on asm.js in particular lately. So you're basically testing Chrome 35 and I'm testing Chrome 40 (which will come through to NW.js soon enough).
I get 1200-1300 objects with box2dweb and 2200+ objects with asm.js here on Chrome, and the dt variations were not much different - in fact it was much more stable at first with asm.js since it could easily reach a steady 60 FPS until the object count started reducing the framerate. The variations may in fact be crbug.com/422000 anyway.