Yes. It is very powerful and even gets the latest Android TV fast 5 years after I bought it.
Yes. I went down a similar road as you, except I also tried Roku, the cheap Chinese android boxes and a Mi Box. I have had a shield in my living room for about two years now and I've rotated many streaming devices in and out of the bedroom. I finally ordered a second shield for the bedroom. It's just so much better than anything else I've tried. Mi Box was OK, but it requires the occasional reboot and it's slow compared to the shield.
What about Dolby Atmos and DTS X support ?
Short answer, yes.
Long answer. It's the best Android based box on the market by far. Ditch the remote though, it's a pile of shit. The tube is only 32 bit iirc, but the full size shield is 64 bit. I don't know what their reasoning is, but I haven't run into any issues using my tube as a result. Chromecast is built in, the menus are fairly responsive, and it's compatible with basically all the major streaming services. I couldn't tell you how the game streaming quality is as I don't use it for that either, but I can tell you it makes a great Emulator box too. 16 bit consoles have no problem running.