Like I said, the AF
mode probably had little to do with the softness of the picture.
The Sigma is a decent enough lens, but at 200mm and above, it is quite soft, especially at lower f-stops. f/5.4 @ 300mm is actually wide open for that lens, and wide open is traditionally where zooms are softest (excluding the other extreme end from f/16+).
You should be able to get good results from it from 70-200mm under good conditions (good lighting, stopped down to f/8 or so), but I wouldn't expect too much from it in the way of sharpness at the long end of the zoom, especially wide open.
If you want to get good results at 200mm and above, you're probably going to need to invest in better glass, as I don't think that even stopping it down will get you much better results @ 300mm.
In short, if you stay in the 70-200mm f/8 range, you should be able to get shots that with a little editing could be accepted here.
If you want any more advice on lenses or shooting techniques, let us know, or you can send me a PM