So I just ordered a Shield tablet, and should have it by sometime next week.
According to someone on the GeForce forums, the WiFi antenna is on the top right section of the picture below:
So when using this tablet in landscape mode (ie. with speakers on the left and right side of the tablet), try to orient it so that the camera is on the
left side. And when using this tablet in portrait mode (ie. with speakers on the top and bottom side of the tablet), try to orient it so that the camera is on the
bottom side of the tablet when not actually using the built in camera for face time.
Note that WiFi signal may become stronger by disabling the "WiFi optimization" setting in the settings power control menu, or simply by rebooting the tablet, but this issue is still being actively investigated by NVIDIA.
Wish me luck