400-800 as a replacement for 200-600?

Hi There,

Im currently without a telezoom and im looking into the 400-800. I had the 200-600 previously but wasn’t that happy with the reach it provided. What i was really happy with was overall image quality.

How does the 400-800 stack up against the 200-600? I mostly shoot little birds, foxes, largest i shoot are Wolfs with around 800-1000 meters of distance.

Thanks!