The Shield autoupdates itself, next major update should be the Marsh Mellow 6.0 update with more features.
You can Root the shield to run Full Android 5.1 with full access to the Play Store and every app available can be installed, apparently though you lose the function of the supplied Game controller but this is easily overcome by using an old Xbox 360 controller
the Hack is here:
[ROM][Full Android for Shield TV][v.2.2] - Post #1244
You can of course also sideload many of the Apps via ESexplorer on the default Shield Build so not a problem.
I would just add this many of the Chinese android market boxes claim many different specifications for 4K there is a standard and all i know is that the Shield TV already has that standard from day one.
For those who dont know the official standard is for Home playback 4K UHD HEVC 10bit through HDMI 2.0 with HDR (high dynamic range) delivered at 60hz
Any device that does not meet that criteria is not full 4K UHD standard, the new Wetek Core is another one it has only just come out and its failed for me anyway as the spec for 4K on the core is HEVC 8bit at 30hz the 30hz part is important as that was the old standard also UHD Blu Ray will be HEVC 10bit @60hz.
anyone thinking of investing in a box the Shield for the price is the only Android box right now that is future proofed
Oh by the way the T8 only does 4K @30hz so its a non starter for me