as a sigma owner, I have to say I'd be surprised if the 150-500 comes anywhere close to the performance of the 100-400, not in terms of AF, but image quality. it's got the harshest bokeh out of any of my lenses and is fairly soft across the board, even when stopped down to f/8.
the sigma 100-300 is a totally different story; light, fast, good range, crisp optics ... too bad there's no OS in it but perhaps sigma will fix that soon