I've been squirrelled away in the PC Gamer office all week testing Halo: Campaign Evolved on all manner of hardware, including a beastly rig with an RTX 5080 and Ryzen 7 7700X3D. To my surprise, the difference between Low and Ultra is incredibly subtle and, frankly, not worth the performance hit.
Across the majority of my testing, I saw the swap from Low to Ultra take a big bite of performance (around 20%). That may not be a huge deal at close to 200 fps, but in my RTX 4070 Super rig, that makes the difference between being able to run the game at a consistent 60 fps at 1440p with no upscaling.