f8 is a bit slow, surprised at the bokeh that you've achieved @ f8. Has good optical quality it seems, matches what others have said about this lens. 2kg isn't too bad for that focal length. I wouldn't recommend hand holding it, even with OS. Get a nice wimberley gyral tripod head (not cheap) and a good tripod.
No AF is perfect (let's look at Canon's Mark III lol). It won't help that you're AF module is basically working at f6.3 - less light for the AF module means slower/less accurate AF. Only way of beating that is paying big moolah for superfast teles like Canon's 500mm f4.
What body?
Dave
edit: if you haven't already, go visit
www.birdsasart.com and grab a copy of Artie's DVD book. Awesome photographer, awesome advice. I rarely bird shoot because I only have a 300mm f4, and even adding a TC to it makes it too short/slow.