I don't care. I have an ultra on an old small HDTV and I think I noticed the overscan before, but I forgot about it, because I don't care about it.
I also have an Ultra on a 4K TV and it has a "fit to screen: on/off" option which eliminates the overscan when on, and eliminates letterboxing when off.
I just tried sending a 4K 30-bit RGB signal to it. It works at 24 Hz, but switches to YCbCr 4:2:0 at 4K60. So an RGB option wouldn't benefit me, but it appears to negotiate what the source and the TV can both handle, IOW, it doesn't hurt. It might aggravate the typical flashing when refresh rates change, though, so I wouldn't want to ever force an RGB preference.
Why don't you put in a feature request to add RGB output if the hardware could accommodate it with a firmware update adding an option in setup? Maybe they can, maybe they can't, maybe they won't for one reason or another. Ask for it, and in the meantime use something else if that's what you've found you prefer.
EDIT: I tried to confirm your results using an onn version of the AndroidTV, GoogleTV/chromecast dongle (not their new $20 item, it's the previous version at around $40 if I recall). I confirmed that an old Proscan 1080P set did overscan a little, using a pattern on a Blu-Ray disc. Connecting the GoogleTV using every combination of 4:4:4, RGB, 24 or 36 bit depths, I get no change. It always overscans the same little bit.
So what you've found may work for your set, but it won't work for mine. Makes me wonder if your set's overscan is much more that mine, or if you're just more sensitive to it. But it doesn't look to me like you found a magic bullet for every set out there. I could be wrong, but that's what I'm seeing this morning.
