Is it really every time? I thought only when battery is low. Surely it takes longer to read/write 8GB, even on SSD, than speed of OS X's sleep/wake cycle.

Unless my understanding is badly flawed, it indeed is every time.

Once you close the lid, the screen goes dark, but the OS does write out the entire sleepimage file every time you sleep.

Yes AFAIK, its written on sleep every time, and only read back if the battery has drained and the RAM contents have been lost during the sleep (so most times you see the fast wake from RAM, and occasionally a tens of seconds wake from disk).

So the part that totally blows me away is that it appears to really write all of the contents of RAM out to the sleep image.

You'd think that the sleep image would omit the memory used for caching, but it doesn't seem to, i have a real 2.0GB file as my sleepimage.

I guess on a hard disk this is probably the correct optimization, but on an SSD it's not.