GH2/100-300 and OIS performance

I have just bought a Panasonic 100-300 and have been trying it out on my GH2.  I took a series of shots of the leaves on an acer tree about 80 yards away at different shutter speeds.

I have found that at 300mm I really need to be at 1/500 for a sharp image even with OIS turned on.  At 1/100 the image is very blurred and still isn't sharp at 1/250.  I have tried OIS mode 1 and mode 2 without any different results although in mode 1 the image is very steady in the viewfinder.

Is this normal?  I thought that the OIS was good for about 3 stops so I would have expected a reasonably sharp image at 1/250 and only a slightly blurred image at 1/100.

I did a comparison with my Canon 7D and a Canon 100-400 lens which has an old Canon IS design that is only really good for 1-2 stops.  400mm on the Canon is roughly the equivalent of 300mm on the Panasonic.

The 100-400 is sharp at both 1/500 and 1/250 and is significantly less blurred than the Panasonic 100-300 at 1/100.

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Chris R