not quite
no longer high end, also not as high powered as you may remember. you can google the exact cpu model(s) and see benchmarks to compare your perceived performance value vs actual.
audio and subtitle transcoding are single threaded, so single core performance is still important.
for full 4k hardware transcoding support, you need quicksync version 6 (Kaby Lake, Coffee Lake, Whiskey Lake) 7xxx series cpu.
pretty much yes, you will still need an atmos receiver if you want hd audio, otherwise you will need to use non-hd audio streams such as dd/dts 5.1 or stereo to avoid audio transcoding.
may also be helpful @ [INFO] Plex, 4k, transcoding, and you - aka the rules of 4k for more info/details.
