I have some issues with the sound :), my HDMI output is plug into my amplifier Yamaha and the iMac with the DP off course.
I didn't find the tips for switch hdmi to DP output. Make the displayport primary? not workd for me.
The last time I get sound I was just lucky
(sorry for my english...
hum that's a good news.
I have just a cable, displayport to miniDp, I hope it works too.
but if your adapter works with your 6850 and not with your 5770, it's a sign :)