No, it does not cause any loss of brightness, but note the cropfactor 1.5. (80-200 --> 120-300)
If it's an old 80-200 f2.8 without built-in AF motor, you won't have autofocus on the 6400.
I have received the Pro version, it would be nice if Digitec could confirm which model they have in stock. As I only use it with old AF-D lenses in manual focus I can't comment on the AF performance