You hand held a 600 mm focal length at 1/30th?? Amazing IS. Does the 50% size stand up ?
Yes, and you can click on the photo (the original one, above) and view the EXIF if you have a viewer - I use FxIF for Firefox - if you don't believe me
My technique for still (or relatively still) critters in low light is to get the shot, then start dropping the ISO until I can't get a sharp photo. ISO 1600 was that point for me, and I also fire off 3 or 4 frames as the shutter speed drops in the hopes of getting at least one sharp frame. My rate was 2/3 frames at 1/30s so yes, the IS (mode 3) was working brilliantly! The owl was up really high so even at 600mm I had to crop a lot (final file is only 1622 x 2433) The 50% zoom holds up decently well - probably good enough for an 8x10" but that's probably the biggest I'd try to print it. I have attached the 50% zoom below.
Really impressive, both the engineering of the lens and your technique !
( I'm off to see which IS version the 300 f2.8 II has )