On a Geforce 9800gt/AMD athlon 64 x2 4400+:
CC: 3967 sprites
C2: Firefox 5: 990, chrome: 119, IE9: 1055
A lot of that is because of the pixel fill rate of the card, though. It gets interesting if you resize the sprite to 12 by 12 and run it again:
CC: 40,000
C2: Firefox 5: 1099, chrome: 1542, IE9: 1324
Also, I think the speed of the behavior/event system is kind of important to check as well. I made my own speed test with the bullet behavior on a 12 x 12 sprite and I get 60 frames per second with about 500 sprites in chrome and IE9.
http://www.amirai.net/forums/C2speedtest.zip
Construct classic gets to about 12,000 sprites before it dips below 60 frames per second. That's a performance difference of about 20:1!
http://www.amirai.net/forums/CCspeedtest.zip
Also, I'm a little confused - you said HTML 5 is supposedly able to be close to c++ speeds (when compiled to machine code), but you also said that it wasn't fast enough to do pixel perfect collisions.
BTW, weird performance on the test I made in Firefox 5 - If I run the test I made with zero sprites, I get 60 frames per second. Creating one 12 x 12 sprite with bullet behavior drops the frames per second to 45, and it can only manage that fps with 100 sprites on screen. I'm guessing that's a Firefox quirk, but I thought I should mention it.