rule of thumb is minimum handhold speed (in seconds) = 1/focal length. the higher magnification it is you shoot at, the easier it is to see effects of camera shake in the picture.
that's why many longer zoom cameras (Canon S1, Nikon 8800, Panasonic FZ20, KM A2) have image stabilization built in.
the a85 doesn't, and it's quite small in size (compared to those cameras i just listed, or a dslr), so it's not so easy to keep it still when you're handholding. so shooting at 300mm you need good light and high shutter speeds (esp since max aperture is not that big on the tele end).
all that said, i haven't used one of those teleconverters, so need to wait for input from those who have one about whether they are useful or not.