I did a benchmark test and posted the results on R0j0hound's chipmunk physics forum post.
These were made using a win7 i5 laptop.
The test, Box containing small bouncing balls with 100% elasticity... I continued to add balls until the cpu usage fluctuated to greater than 85%
Something to note here which is very important. AMS.JS Keeps low cpu usage and then once you cross a object count it sky rockets to 90-100% . AMS.JS seems to run an average lower cpu usage than regular Box2d web, but I am notice high cpu spikes. If ams.js spikes to be at where Box2d web runs or even above, I am not sure its benifits are worth it.
When I am making a game, I really prefer to have predictable results and right now ams.js has some unpredictability. If you end up using 500 objects and get frequent spikes, then game play could be affected in a variable way (slow, smooth,slow,smooth). If Ams spike to where box2d web runs at then you really can only safely use the same amount of objects. Does anyone have some knowledge of what is happening here?
Box2d is much more linear in its object count to cpu usage. Chipmunk starts to use more cpu as the balls settle (which they shouldn't be doing). The settling becomes more pronounced as the object count increases.