f/8 is the sweet spot (not sure how you even get to f/9).
I've been real pleased with my copy. Check out my flickr link if you want to see some pics. Unfortunately, after Christmas I'm going to send in into Canon for repair. Its suddenly become vary soft at 300mm with some strange haloing especially with with anything white. I think something is out of alignment.
Bummer about the softness and halo. Hope it's fixed soon.