I guess i shouldn't be suprised with only a small amount of answers for you (& me). Perhaps you should also post the question over a www.6mmbr.com
on the forums for perhaps a better response.
I think the short answer is - if the weather is right (not too windy), target is ranged & compensated for (dial in the scope with accurate, tested drop chart)
and with a good rest 600 yards is doable with a good "nut behind the butt"
A local F-classer shooter who upon hearing I were building a 6mmBR for hunting simply "that will do just fine to 600yards". Incidently he is a very capable f-class shooter who is very highly ranked and yes shoots the 6mmBR.
I be posting some field results hopefully not it the too far future.