I have done a lot of research about this issue and I found out that it purely rely on the TV HDMI-ARC passthrough to determine whether you can have the 5.1 surround option in Netflix app on the shield TV. So the result will be vary based on the TV models.
I just want to let you know that it is now possible to choose 5.1 surround option in Netflix after upgrading to the SHIELD Experience 8.0. As long as you manually choose to enable the dolby digital plus format under advanced sound settings. It worked in my test.
I am not familiar with these technical explanations so please forgive me if I use the wrong terms.