It's venturing into cryptocurrency-space-heater levels of pointless number crunching to render at that level of detail for anyone who has human eyes.
Depends on your monitor size.
Might be a waste at 27", but if you want to use a 48" display, I can assure you that you'd notice the move from 4k -> 8k.
8K in a game will use tremendous amounts of power, but it's not pointless. It's like having antialiasing turned on. And high frame rates are important for motion because normal rendering only gives you a single point in time.