The ARC should be capable of returning 5.1 audio from HDMI 1.4 compatible devices. I dont think higher quality 7.1 audio formats are included in the HDMI 1.4 ARC specification. I think some of the confusion is from many posts from people who could not get ARC to work reliably (if at all) with certain AV receivers that may have been HDMI 1.3 units or just didnt play well with other components. When that failed, some connected an optical cable from the TV's audio out to the receiver.
The optical out will dumb down the audio to stereo for sure. If you want to maintain and use the advanced audio formats on BlueRay you will need to connect the individual components to the receiver. If you are OK with 5.1 then the ARC may work. HDMaster says no, but ...... Just try it. If you get 5.1 audio then its working like ARC was intended to. If you only get stereo, then HDMaster is correct and Panasonic did not fully implement the HDMI 1.4 ARC standard.
Also, if a Panasonic sales rep told you the GT50 was a 4K TV he should be fired for either lying or being an idiot. A very quick look at the specs for the TV at any of a dozen web sites would have quickly confirmed its a 1080P model just like nearly all other TV's available today. Dont feel to bad about it since there wont be any substantial amount of 4K content for years.
EDIT ... I just dug around the net; looks like Samsung and LG both dumd down ARC to stereo. Hard to beleive Panasonic would do anything different. I'm guessing HDMaster is correct and any of the nonsense coming from Panasonic is not accurate. Big surprise Edited by tpollagi - 10/2/13 at 8:15pm