When you say stream at 2160x2160, where is that configured?If we're talking about the Pico 4U, yes it can stream at 2160x2160, it took some fettling but for some reason they didn't enable it out of the box - weird. I can even stream at any resolution above that and let the headset down sample. This is using the Pico 4U connect software.
With the different headsets and technology, I am inclined to always use whatever resolution the computer is generating frames at.
My headset is SteamVR native so the SteamVR resolution is what I use as a reference, assuming I am leaving HMD quality and super sampling to 1.0
I actually wonder what other software would show that for a headset and game not using SteamVR.