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Had to make a setting decision and went with keeping the ISO as low as possible and with IS turned on went to the slowest shutter speed I was comfortable using. I was in a setting position with elbows resting on my knees for support
Although the sharpness looks great here I would not hesitate one bit in upping the ISO with your 40D, just to put chances on your side more consistently. I have the same camera/lens combo as you do and I don't worry at all about noise issues at all if I have to raise it. P.S. good "stabilizing" technique.