RK3399 in mainline linux is rather good as it is also used in some Chromebooks (OP1 processor = RK3399).
When it comes to the 4.4 kernel rockchip have a pretty recent Linaro Stable Kernel (Android) merged, unfortunately when I asked if there was a chance to see a new version pushed to github it ended with "not a chance", I am speculating that the internal rockchip linux tree contains code for unreleased products blocking pushing a new version public.
For LE we use a special 4.4 version that I have tuned for video/audio/cec/media, see the different rockchip-4.4 branches at Branches · Kwiboo/linux-rockchip · GitHub, they correspond to the patches seen at rockchip-4.4, the base version used in LE corresponds to the release-4.4 branch in my kernel tree.
Moving forward I am mainly focused on mainline linux development (rk3288 and rk3328 already works rather okay), my next step is to confirm rk3399 devices works before I push initial mainline support to LE.
As mo123 mentions gpu driver exists for T860, you can find it at GitHub - rockchip-linux/libmali: The Mali GPU library used in Rockchip Platform, for LE we only use the gbm version.
When it comes to tv boxes I would also highly recommend any of the SBC type of devices, box devices will not see much support when we move to mainline unless vendor or other community members upstream device tree to mainline u-boot/linux.