Just get Lan usb hub... ur will not just get lan ur also get extra usb port.

Remember mi box only support usb 2.0

What Puchongite says.. HW is great for comparisons and narrowing down one's minimal specs requirements. But after that? Software plays a heavy part. I've seen devices with 4GB RAM, faster SoC, etc perform poorly compared to some 2GB RAM with older SoC devices from solid manufacturers. Though is support is a major concern of yours? You are better of with the MiBox.
No.. There's a point. While USB3.0 is backwards compatible. A USB2.0 port can magically have 3.0 speeds. Similarly, watching 4K content on a 720p display. Sure the image will look sharper but no where near the true 4K quality as would on a proper 4K display.

Android boxes depends 100 or 1000 mbps

Personally 100mbps box is more than suffice even for 4k streaming