Just my opinions:
Long range subsonic shooting still requires low drag bullets. That's why the 300 Whisper is popular in the AR-15. It's about the heaviest low drag cartridge/bullet which can pass though an AR-15 magazine. The 338 Spectre can't handle the heaviest 338 bullets in an AR-15 magazine.
I own a 458 SOCOM RRA ar-15, and i don't consider it suitable for subsoinic shooting. It's fine as a short range hunting or urban entry rifle for bullets in the 90 to 350 grain range with standard supersonic loads.
The 458 SOCOM can be a great long range subsonic cartridge, but it needs at least a 30-06 length action and magazine. The Lehigh 582 grain 458 bullet is a good subsonic match for the case capacity of the 458 SOCOM That's the fifth cartridge in this picture:
